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{{For|the academic journal|Systematic Reviews (journal)}}
{{For|the academic journal|Systematic Reviews (journal)}}
{{short description|Comprehensive review of research literature using systematic methods}}
{{short description|Comprehensive review of research literature using systematic methods}}
{{Research}}'''Systematic reviews''' are a type of [[Literature review|review]] that uses repeatable analytical methods to collect secondary data and analyse it. Systematic reviews are a type of evidence synthesis which formulate research questions that are broad or narrow in scope, and identify and synthesize data that directly relate to the systematic review question.<ref>{{cite web|title=systematic review|url=http://getitglossary.org/term/systematic+review|access-date=18 November 2015|website=GET-IT glossary}}</ref> While some people might associate ‘systematic review’ with 'meta-analysis', there are multiple kinds of review which can be defined as ‘systematic’ which do not involve a meta-analysis. Some systematic reviews critically appraise research studies, and synthesize findings qualitatively or quantitatively.<ref>{{cite journal|vauthors=Armstrong R, Hall BJ, Doyle J, Waters E|date=March 2011|title=Cochrane Update. 'Scoping the scope' of a cochrane review|journal=Journal of Public Health|volume=33|issue=1|pages=147–50|doi=10.1093/pubmed/fdr015|pmid=21345890}}</ref> Systematic reviews are often designed to provide an exhaustive summary of current evidence relevant to a [[research question]]. For example, systematic reviews of [[Randomized controlled trial|randomized controlled trials]] are an important way of informing [[evidence-based medicine]],<ref name="CEBM_about2">{{cite web|date=2009-11-20|title=What is EBM?|url=http://www.cebm.net/index.aspx?o=1914|archive-url=https://web.archive.org/web/20110406110628/http://www.cebm.net/index.aspx?o=1914|archive-date=2011-04-06|access-date=2011-06-17|publisher=Centre for Evidence Based Medicine}}</ref> and a review of existing studies is often quicker and cheaper than embarking on a new study.
{{Research}}


While systematic reviews are often applied in the [[Biomedical research|biomedical]] or healthcare context, they can be used in other areas where an assessment of a precisely defined subject would be helpful.<ref>{{cite book|last1=Ader|first1=Herman J.|title=Advising on Research Methods: A consultant's companion|last2=Mellenbergh|first2=Gideon J.|last3=Hand|first3=David J.|publisher=Johannes van Kessel Publishing|year=2008|isbn=978-90-79418-02-2|chapter=Methodological quality|name-list-format=vanc|chapter-url=https://books.google.com/books?id=LCnOj4ZFyjkC&printsec=frontcover&hl=fr#v=onepage&q=%22Methodological%20quality%22&f=false}}</ref> Systematic reviews may examine clinical tests, public health interventions, environmental interventions,<ref>{{cite journal|last1=Bilotta|first1=Gary S.|last2=Milner|first2=Alice M.|last3=Boyd|first3=Ian|year=2014|title=On the use of systematic reviews to inform environmental policies|journal=Environmental Science & Policy|volume=42|pages=67–77|doi=10.1016/j.envsci.2014.05.010|name-list-format=vanc}}</ref> social interventions, [[adverse effects]], qualitative evidence syntheses, methodological reviews, policy reviews, and [[Economic evaluation|economic evaluations]].<ref name="CRD20082">{{cite book|url=https://www.york.ac.uk/media/crd/Systematic_Reviews.pdf|title=Systematic reviews: CRD's guidance for undertaking reviews in health care.|date=2008|publisher=University of York, Centre for Reviews and Dissemination|isbn=978-1-900640-47-3|location=York|access-date=17 June 2011}}</ref><ref>{{cite book|url=http://www.cebma.org/wp-content/uploads/Pettigrew-Roberts-SR-in-the-Soc-Sc.pdf|title=Systematic reviews in the social sciences|vauthors=Petticrew M, Roberts H|date=2006|publisher=Wiley Blackwell|isbn=978-1-4051-2110-1|archive-url=https://web.archive.org/web/20150616034557/http://www.cebma.org/wp-content/uploads/Pettigrew-Roberts-SR-in-the-Soc-Sc.pdf|archive-date=2015-06-16}}</ref>
A '''systematic review''', or '''systematic literature review''', is a type of [[literature review]] that uses systematic methods to collect secondary data, critically appraise research studies, and synthesize findings qualitatively or quantitatively.<sup><ref>{{cite journal|vauthors=Armstrong R, Hall BJ, Doyle J, Waters E|date=March 2011|title=Cochrane Update. 'Scoping the scope' of a cochrane review|journal=Journal of Public Health|volume=33|issue=1|pages=147–50|doi=10.1093/pubmed/fdr015|pmid=21345890|doi-access=free}}</ref></sup> Systematic reviews formulate [[research question]]s that are broad or narrow in scope, and identify and synthesize studies that directly relate to the systematic review question.<ref>{{cite web|url=http://getitglossary.org/term/systematic+review|title=systematic review|website=GET-IT glossary|access-date=18 November 2015}}</ref> They are designed to provide a complete, exhaustive summary of current evidence, published and unpublished, that is "methodical, comprehensive, transparent, and replicable."<ref>{{Cite journal|last1=Siddaway|first1=Andy P.|last2=Wood|first2=Alex M.|last3=Hedges|first3=Larry V.|date=2019-01-04|title=How to Do a Systematic Review: A Best Practice Guide for Conducting and Reporting Narrative Reviews, Meta-Analyses, and Meta-Syntheses|journal=Annual Review of Psychology|language=en|volume=70|issue=1|pages=747–770|doi=10.1146/annurev-psych-010418-102803|pmid=30089228|issn=0066-4308}}</ref>{{rp|751}}


An understanding of systematic reviews and how to implement them in practice is highly recommended for professionals involved in the delivery of [[health care]]<nowiki>, public health and public policy. }}</nowiki>
An understanding of systematic reviews and how to implement them in practice is highly recommended for professionals involved in the delivery of [[health care]], public health, and public policy.<ref name="Giang">{{cite journal |last1=Giang |first1=Hoang Thi Nam |last2=Ahmed |first2=Ali Mahmoud |last3=Fala |first3=Reem Yousry |last4=Khattab |first4=Mohamed Magdy |last5=Othman |first5=Mona Hassan Ahmed |last6=Abdelrahman |first6=Sara Attia Mahmoud |last7=Thao |first7=Le Phuong |last8=Gabl |first8=Ahmed Elsaid Abd Elsamie |last9=Elrashedy |first9=Samar Ahmed |last10=Lee |first10=Peter N. |last11=Hirayama |first11=Kenji |last12=Salem |first12=Hosni |last13=Huy |first13=Nguyen Tien |title=Methodological steps used by authors of systematic reviews and meta-analyses of clinical trials: a cross-sectional study |journal=BMC Medical Research Methodology |date=26 July 2019 |volume=19 |issue=1 |pages=164 |doi=10.1186/s12874-019-0780-2 |pmid=31349805 |pmc=6659247 |name-list-style=vanc}}</ref> Systematic reviews of [[randomized controlled trial]]s are key to the practice of [[evidence-based medicine]],<ref name="CEBM_about">{{cite web|url=http://www.cebm.net/index.aspx?o=1914|title=What is EBM?|date=2009-11-20|publisher=[[Centre for Evidence Based Medicine]]|archive-url=https://web.archive.org/web/20110406110628/http://www.cebm.net/index.aspx?o=1914|archive-date=2011-04-06|access-date=2011-06-17}}</ref><ref>{{cite journal |last1=Farag |first1=Sara M. |last2=Mohammed |first2=Manal O. |last3=EL-Sobky |first3=Tamer A. |last4=ElKadery |first4=Nadia A. |last5=ElZohiery |first5=Abeer K. |title=Botulinum Toxin A Injection in Treatment of Upper Limb Spasticity in Children with Cerebral Palsy: A Systematic Review of Randomized Controlled Trials |journal=JBJS Reviews |date=March 2020 |volume=8 |issue=3 |pages=e0119 |doi=10.2106/JBJS.RVW.19.00119 |pmid=32224633|pmc=7161716 |doi-access=free }}</ref> and a review of existing studies is often quicker and cheaper than embarking on a new study. Contrastingly, systematic reviews of observational studies rank lower in the evidence-based hierarchy. However, another important factor that impacts the quality of the evidence is the accuracy of the methodological design and execution of the systematic review carried out by the authors.

While systematic reviews are often applied in the [[Biomedical research|biomedical]] or healthcare context, they can be used in other areas where an assessment of a precisely defined subject would be helpful.<ref name="Herman2008">{{cite book|title=Advising on Research Methods: A consultant's companion|last1=Ader|first1=Herman J.|last2=Mellenbergh|first2=Gideon J.|last3=Hand|first3=David J.|publisher=Johannes van Kessel Publishing|year=2008|isbn=978-90-79418-02-2|chapter=Methodological quality|name-list-style=vanc|authorlink1=Herman J. Adèr|authorlink2=Gideon J. Mellenbergh|chapter-url=https://books.google.com/books?id=LCnOj4ZFyjkC}}{{page needed|date=June 2012}}</ref> For example, systematic reviews are becoming increasingly common in [[management]], [[accounting]] and [[finance]].<ref>{{Cite journal|last1=Linnenluecke|first1=Martina K|last2=Marrone|first2=Mauricio|last3=Singh|first3=Abhay K|date=2019-10-03|title=Conducting systematic literature reviews and bibliometric analyses|journal=Australian Journal of Management|language=en-US|volume=45|issue=2|pages=175–194|doi=10.1177/0312896219877678|s2cid=211378937|issn=0312-8962}}</ref> Systematic reviews may examine clinical tests, public health interventions, environmental interventions,<ref>{{cite journal|last1=Bilotta|first1=Gary S.|last2=Milner|first2=Alice M.|last3=Boyd|first3=Ian|year=2014|title=On the use of systematic reviews to inform environmental policies|journal=Environmental Science & Policy|volume=42|pages=67–77|doi=10.1016/j.envsci.2014.05.010|name-list-style=vanc|doi-access=free}}</ref> social interventions, [[adverse effects]], and [[economic evaluation]]s.<ref name="CRD2008">{{cite book|url=https://www.york.ac.uk/media/crd/Systematic_Reviews.pdf|title=Systematic reviews: CRD's guidance for undertaking reviews in health care.|date=2008|publisher=[[University of York]], Centre for Reviews and Dissemination|isbn=978-1-900640-47-3|location=York|access-date=17 June 2011}} {{page needed|date=June 2012}}</ref><ref>{{cite book|url=http://www.cebma.org/wp-content/uploads/Pettigrew-Roberts-SR-in-the-Soc-Sc.pdf|title=Systematic reviews in the social sciences|vauthors=Petticrew M, Roberts H|date=2006|publisher=Wiley Blackwell|isbn=978-1-4051-2110-1|archive-url=https://web.archive.org/web/20150616034557/http://www.cebma.org/wp-content/uploads/Pettigrew-Roberts-SR-in-the-Soc-Sc.pdf|archive-date=2015-06-16}}</ref>


==Characteristics==
==Characteristics==
Systematic reviews can be used to inform decision making in many different disciplines, such as evidence-based healthcare and evidence-based policy and practice.<ref name=":0" />
A systematic review aims to provide a complete, exhaustive summary of current literature relevant to a [[research question]]. The first step in conducting a systematic review is to create a structured question to guide the review.<ref>{{cite book | vauthors = Eden J, Levit L, Berg A, Morton S | collaboration = Institute of Medicine (US) Committee on Standards for Systematic Reviews of Comparative Effectiveness Research |title= Finding What Works in Health Care: Standards for Systematic Reviews. |date=2011 |pmid=24983062 |doi=10.17226/13059 |isbn=978-0-309-16425-2 | editor1-last = Eden | editor1-first = Jill | editor2-last = Levit | editor2-first = Laura | editor3-last = Berg | editor3-first = Alfred | editor4-last = Morton | editor4-first = Sally }}</ref> The second step is to perform a thorough search of the literature for relevant papers. The ''Methodology'' section of a systematic review will list all of the [[database]]s and [[citation index]]es that were searched such as [[Web of Science]], [[Embase]], and [[PubMed]] and any individual journals that were searched. The searching of different databases is a hallmark of clinical trials. In this regard, more recently there has been increased recognition of the importance of using different search technologies, with artificial intelligence-based tools gaining recognition. The titles and abstracts of identified articles are checked against pre-determined criteria for eligibility and relevance to form an inclusion set. This set will relate back to the research problem. Each included study may be assigned an objective assessment of methodological quality preferably by using methods conforming to the [[Preferred Reporting Items for Systematic Reviews and Meta-Analyses]] (PRISMA) statement (the current guideline)<ref>{{cite web|url=http://www.prisma-statement.org/ |title=PRISMA |publisher=Prisma-statement.org |access-date=2013-08-29}}</ref> or the high quality standards of Cochrane.<ref name=CochraneHandbook>{{cite book |date=March 2011 |url=http://handbook.cochrane.org | veditors = Higgins JP, Green S |title=Cochrane Handbook for Systematic Reviews of Interventions |edition=Version 5.1 |publisher=The Cochrane Collaboration}}</ref>


A systematic review can be designed to provide an exhaustive summary of current literature relevant to a research question.
Systematic reviews often, but not always, use statistical techniques ([[meta-analysis]]) to combine results of eligible studies, or at least use scoring of the levels of evidence depending on the methodology used. An additional rater may be consulted to resolve any scoring differences between raters.<ref name="Herman2008"/> Systematic review is often applied in the [[biomedical research|biomedical]] or healthcare context, but it can be applied in any field of research. Groups like the [[Campbell Collaboration]] are promoting the use of systematic reviews in policy-making beyond just healthcare.


A systematic review uses an objective and transparent approach for research synthesis, with the aim of minimizing bias. While many systematic reviews are based on an explicit [[quantitative research|quantitative]] meta-analysis of available data, there are also [[qualitative research|qualitative]] reviews which adhere to standards for gathering, analyzing and reporting evidence.<ref name=Bearman2013>{{cite journal | vauthors = Bearman M, Dawson P | title = Qualitative synthesis and systematic review in health professions education | journal = Medical Education | volume = 47 | issue = 3 | pages = 252–60 | date = March 2013 | pmid = 23398011 | doi = 10.1111/medu.12092 }}</ref> The [[EPPI-Centre]] has been influential in developing methods for combining both qualitative and quantitative research in systematic reviews.<ref>{{cite journal | vauthors = Thomas J, Harden A, Oakley A, Oliver S, Sutcliffe K, Rees R, Brunton G, Kavanagh J | title = Integrating qualitative research with trials in systematic reviews | journal = BMJ | volume = 328 | issue = 7446 | pages = 1010–2 | date = April 2004 | pmid = 15105329 | pmc = 404509 | doi = 10.1136/bmj.328.7446.1010 }}</ref> The PRISMA statement<ref>{{cite journal | vauthors = Liberati A, Altman DG, Tetzlaff J, Mulrow C, Gøtzsche PC, Ioannidis JP, Clarke M, Devereaux PJ, Kleijnen J, Moher D | title = The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: explanation and elaboration | journal = PLOS Medicine | volume = 6 | issue = 7 | pages = e1000100 | date = July 2009 | pmid = 19621070 | pmc = 2707010 | doi = 10.1371/journal.pmed.1000100 }}</ref> suggests a standardized way to ensure a transparent and complete reporting of systematic reviews, and is now required for this kind of research by more than 170 medical journals worldwide.<ref>{{cite web |url=http://www.prisma-statement.org/Endorsement/PRISMAEndorsers.aspx |title=PRISMA Endorsers |website=www.prisma-statement.org|access-date=1 June 2016}}</ref>
A systematic review uses a rigorous and transparent approach for research synthesis, with the aim of assessing and, where possible, minimizing bias in the findings. While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews and other types of mixed-methods reviews which adhere to standards for gathering, analyzing and reporting evidence.<ref>{{Cite journal|last=Bearman|first=Margaret|last2=Dawson|first2=Phillip|date=2013|title=Qualitative synthesis and systematic review in health professions education|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/medu.12092|journal=Medical Education|language=en|volume=47|issue=3|pages=252–260|doi=10.1111/medu.12092|issn=1365-2923}}</ref>


Systematic reviews of quantitative data or mixed-method reviews sometimes use statistical techniques (meta-analysis) to combine results of eligible studies. Scoring levels are sometimes used to rate the quality of the evidence depending on the methodology used, although this is discouraged by the Cochrane Library.<ref>{{Cite book|url=https://training.cochrane.org/handbook/current|title=Cochrane Handbook for Systematic Reviews of Interventions|date=2019-09-20|website=training.cochrane.org|editor-last=Higgins|editor-first=Julian P.T.|series=version 6.1|location=|pages=section 4.6|language=en|access-date=2020-09-14|editor2-last=Thomas|editor2-first=James|editor3-last=Chandler|editor3-first=Jacqueline|editor4-last=Cumpston|editor4-first=Miranda|editor5-last=Li|editor5-first=Tianjing|editor6-last=Page|editor6-first=Matthew J.|editor7-last=Welch|editor7-first=Vivian A.}}</ref> As evidence rating can be subjective, multiple people may be consulted to resolve any scoring differences between how evidence is rated.<ref>{{Cite web|last=Siemieniuk|first=Reed|last2=Guyatt|first2=Gordon|date=|title=What is GRADE?|url=https://bestpractice.bmj.com/info/toolkit/learn-ebm/what-is-grade/|archive-url=|archive-date=|access-date=2020-08-26|website=BMJ Best Practice|language=en}}</ref><ref>{{Cite book|last=Adèr|first=Hermanus Johannes|url=https://books.google.com/books?id=LCnOj4ZFyjkC&newbks=0&hl=en|title=Advising on Research Methods: A Consultant's Companion|date=2008|publisher=Johannes van Kessel Publishing.|isbn=978-90-79418-01-5|language=en}}</ref><ref>{{Cite book|last=|first=|url=https://training.cochrane.org/resource/grade-handbook|title=GRADE Handbook|publisher=|year=2013|isbn=|editor-last=Schünemann|editor-first=Holger|location=|pages=|language=en|access-date=2020-08-26|editor-last2=Brożek|editor-first2=Jan|editor-last3=Guyatt|editor-first3=Gordon|editor-last4=Oxman|editor-first4=Andrew}}</ref>
Developments in systematic reviews during the 21st century included [[Philosophical realism|realist]] reviews and the [[meta-narrative]] approach, both of which addressed problems of methods and [[heterogeneity]] existing on some subjects.<ref>{{cite journal | vauthors = Pawson R, Greenhalgh T, Harvey G, Walshe K | title = Realist review--a new method of systematic review designed for complex policy interventions | journal = Journal of Health Services Research & Policy | volume = 10 Suppl 1 | pages = 21–34 | date = July 2005 | pmid = 16053581 | doi = 10.1258/1355819054308530 | s2cid = 9133437 }}</ref><ref>{{cite journal | vauthors = Greenhalgh T, Potts HW, Wong G, Bark P, Swinglehurst D | title = Tensions and paradoxes in electronic patient record research: a systematic literature review using the meta-narrative method | journal = The Milbank Quarterly | volume = 87 | issue = 4 | pages = 729–88 | date = December 2009 | pmid = 20021585 | pmc = 2888022 | doi = 10.1111/j.1468-0009.2009.00578.x | jstor = 25593645 }}</ref>

The [[wikipedia:EPPI-Centre|EPPI-Centre]], [[wikipedia:Cochrane_(organisation)|Cochrane]] and the [[wikipedia:The_Joanna_Briggs_Institute|Joanna Briggs Institute]] have all been influential in developing methods for combining both qualitative and quantitative research in systematic reviews.<ref name=":1">{{Cite journal|last=Flemming|first=Kate|last2=Booth|first2=Andrew|last3=Hannes|first3=Karin|last4=Cargo|first4=Margaret|last5=Noyes|first5=Jane|date=2018-05|title=Cochrane Qualitative and Implementation Methods Group guidance series—paper 6: reporting guidelines for qualitative, implementation, and process evaluation evidence syntheses|url=https://linkinghub.elsevier.com/retrieve/pii/S0895435617313276|journal=Journal of Clinical Epidemiology|language=en|volume=97|pages=79–85|doi=10.1016/j.jclinepi.2017.10.022}}</ref><ref>{{Cite journal|last=Harden|first=Angela|last2=Thomas|first2=James|last3=Cargo|first3=Margaret|last4=Harris|first4=Janet|last5=Pantoja|first5=Tomas|last6=Flemming|first6=Kate|last7=Booth|first7=Andrew|last8=Garside|first8=Ruth|last9=Hannes|first9=Karin|date=2018-05-01|title=Cochrane Qualitative and Implementation Methods Group guidance series—paper 5: methods for integrating qualitative and implementation evidence within intervention effectiveness reviews|url=http://www.sciencedirect.com/science/article/pii/S0895435617313549|journal=Journal of Clinical Epidemiology|language=en|volume=97|pages=70–78|doi=10.1016/j.jclinepi.2017.11.029|issn=0895-4356}}</ref><ref>{{Cite web|title=EPPI-Centre Home|url=https://eppi.ioe.ac.uk/cms/|access-date=2020-06-29|website=eppi.ioe.ac.uk}}</ref> Several reporting guidelines exist to standardise reporting about how systematic reviews are conducted. Such reporting guidelines are not quality assessment or appraisal tools. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement<ref name=":2" /> suggests a standardized way to ensure a transparent and complete reporting of systematic reviews, and is now required for this kind of research by more than 170 medical journals worldwide.<ref name=":0" /> Several specialized PRISMA guideline extensions have been developed to support particular types of studies or aspects of the review process, including PRISMA-P for review protocols and PRISMA-ScR for scoping reviews.<ref name=":0" /> A list of PRISMA guideline extensions is hosted by the EQUATOR (Enhancing the QUAlity and Transparency Of health Research) Network.<ref name=":5">{{Cite web|last=|first=|date=|title=Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement|url=https://www.equator-network.org/reporting-guidelines/prisma/|archive-url=|archive-date=|access-date=2020-06-29|website=www.equator-network.org}}</ref>

For qualitative reviews, reporting guidelines include ENTREQ (Enhancing transparency in reporting the synthesis of qualitative research) for qualitative evidence syntheses; RAMESES (Realist And MEta-narrative Evidence Syntheses: Evolving Standards) for meta-narrative and realist reviews;<ref>{{Cite journal|last=Wong|first=Geoff|last2=Greenhalgh|first2=Trish|last3=Westhorp|first3=Gill|last4=Buckingham|first4=Jeanette|last5=Pawson|first5=Ray|date=2013|title=RAMESES publication standards: meta‐narrative reviews|url=https://onlinelibrary.wiley.com/doi/10.1111/jan.12092|journal=[[w:Journal of Advanced Nursing|Journal of Advanced Nursing]]|language=en|volume=97|pages=987–1004|doi=10.1111/jan.12092}}</ref><ref>{{Cite journal|last=Wong|first=Geoff|last2=Greenhalgh|first2=Trish|last3=Westhorp|first3=Gill|last4=Buckingham|first4=Jeanette|last5=Pawson|first5=Ray|date=2013|title=RAMESES publication standards: realist syntheses|url=https://onlinelibrary.wiley.com/doi/10.1111/jan.12095|journal=[[w:Journal of Advanced Nursing|Journal of Advanced Nursing]]|language=en|volume=97|pages=987–1004|doi=10.1111/jan.12095}}</ref> and eMERGe (Improving reporting of Meta-Ethnography) for meta-[[wikipedia:Ethnography|ethnograph]].<ref name=":1" />

Developments in systematic reviews during the 21st century included realist reviews and the meta-narrative approach, both of which addressed problems of variation in methods and heterogeneity existing on some subjects.<ref name=":3" /><ref name=":4" />

== Types of systematic review ==
There are over 30 types of systematic review and the '''Table 1''' below summarises some of these, but it is not exhaustive.<ref name=":0">{{Cite journal|last=Grant|first=Maria J.|last2=Booth|first2=Andrew|date=2009|title=A typology of reviews: an analysis of 14 review types and associated methodologies|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1471-1842.2009.00848.x|journal=Health Information & Libraries Journal|language=en|volume=26|issue=2|pages=91–108|doi=10.1111/j.1471-1842.2009.00848.x|issn=1471-1842}}</ref><ref name=":2">{{Cite book|last1=Booth|first1=A|url=http://worldcat.org/oclc/944453327|title=Guidance on choosing qualitative evidence synthesis methods for use in health technology assessments of complex interventions|last2=Noyes|first2=J|last3=Flemming|first3=K|last4=Gerhardus|first4=A|last5=Wahlster|first5=P|last6=Van der Wilt|first6=GJ|last7=Mozygemba|first7=K|last8=Refolo|first8=P|last9=Sacchini|first9=D|date=2016|publisher=|year=|isbn=|location=|page=32|pages=|oclc=944453327|last10=Tummers|first10=M|last11=Rehfuess|first11=E}}</ref> It is important to note that there is not always consensus on the boundaries and distinctions between the approaches described below.
{| class="wikitable"
|+Table 1: A summary of some of the types of systematic review.
!Review type
!Summary
|-
|Mapping review/systematic map
|A mapping review maps existing literature and categorizes data. The method characterizes quantity and quality of literature, including by study design and other features. Mapping reviews can be used to identify the need for primary or secondary research.<ref name=":0" />
|-
|Meta-analysis
|A meta-analysis is a statistical analysis that combines the results of multiple quantitative studies. Using statistical methods, results are combined to provide evidence from multiple studies. The two types of data generally used for meta-analysis in health research are individual participant data and aggregate data (such as odds ratios or relative risks).
|-
|Mixed studies review/mixed methods review
|Refers to any combination of methods where one significant stage is a literature review (often systematic). It can also refer to a combination of review approaches such as combining quantitative with qualitative research.<ref name=":0" />
|-
|Qualitative systematic review/qualitative evidence synthesis
|This method for integrates or compares findings from qualitative studies. The method can include ‘coding’ the data and looking for ‘themes’ or ‘constructs’ across studies. Multiple authors may improve the ‘validity’ of the data by potentially reducing individual bias.<ref name=":0" />
|-
|Rapid review
|An assessment of what is already known about a policy or practice issue, which uses systematic review methods to search for and critically appraise existing research. Rapid reviews are still a systematic review, however parts of the process may be simplified or omitted in order to increase rapidity.<ref>{{Cite web|title=What is a rapid review? Systematic Review Library Guides at CQ University|url=https://libguides.library.cqu.edu.au/c.php?g=849703&p=6154326|archive-url=https://web.archive.org/web/20200916051435/https://libguides.library.cqu.edu.au/c.php?g=849703&p=6154326|archive-date=2020-09-16|access-date=2020-09-16|website=library.cqu.edu.au}}</ref> Rapid reviews were used during the COVID-19 pandemic.<ref>{{Cite web|title=Home|url=https://covidrapidreviews.cochrane.org/welcome|access-date=2020-07-01|website=covidrapidreviews.cochrane.org|language=en}}</ref>
|-
|Systematic review
|A systematic search for data, using a repeatable method. It includes appraising the data (for example the quality of the data) and a synthesis of research data.
|-
|Systematic search and review
|Combines methods from a ‘critical review’ with a comprehensive search process. This review type is usually used to address broad questions to produce the most appropriate evidence synthesis. This method may or may not include quality assessment of data sources.<ref name=":0" />
|-
|Systematized review
|Include elements of systematic review process, but searching is often not as comprehensive as a systematic review and may not include quality assessments of data sources.
|}


===Scoping reviews===
===Scoping reviews===
Scoping reviews are distinct from systematic reviews in several important ways. A scoping review is an attempt to search for concepts by mapping the language and data which surrounds those concepts and adjusting the search method iteratively to synthesize evidence and assess the scope of an area of inquiry.<ref name=":3">{{cite journal|last1=Arksey|first1=Hilary|last2=O'Malley|first2=Lisa|year=2005|title=Scoping studies: Towards a methodological framework|journal=International Journal of Social Research Methodology|volume=8|pages=19–32|doi=10.1080/1364557032000119616|name-list-format=vanc}}</ref><ref name=":4">{{Cite web|title=PRISMA|url=http://www.prisma-statement.org/Extensions/ScopingReviews|access-date=2020-07-01|website=www.prisma-statement.org}}</ref> This can mean that the concept search and method (including data extraction, organisation and analysis) are refined throughout the process, sometimes requiring deviations from any protocol or original research plan.<ref name="Peters 141–146">{{Cite journal|last=Peters|first=Micah D. J.|last2=Godfrey|first2=Christina M.|last3=Khalil|first3=Hanan|last4=McInerney|first4=Patricia|last5=Parker|first5=Deborah|last6=Soares|first6=Cassia Baldini|date=2015-09|title=Guidance for conducting systematic scoping reviews|url=https://journals.lww.com/ijebh/fulltext/2015/09000/Guidance_for_conducting_systematic_scoping_reviews.5.aspx|journal=JBI Evidence Implementation|language=en|volume=13|issue=3|pages=141–146|doi=10.1097/XEB.0000000000000050|issn=2691-3321}}</ref><ref>{{Cite journal|last=Levac|first=Danielle|last2=Colquhoun|first2=Heather|last3=O'Brien|first3=Kelly K|date=2010-12|title=Scoping studies: advancing the methodology|url=http://implementationscience.biomedcentral.com/articles/10.1186/1748-5908-5-69|journal=Implementation Science|language=en|volume=5|issue=1|pages=69|doi=10.1186/1748-5908-5-69|issn=1748-5908}}</ref> A [[scoping review]] may often be a preliminary stage before a systematic review, which 'scopes' out an area of inquiry and maps the language and key concepts to determine if a systematic review is possible or appropriate, or to lay the groundwork for a full systematic review. The goal can be to assess how much data or evidence is available regarding a certain area of interest.<ref name="Peters 141–146" /><ref>{{Cite journal|last=Colquhoun|first=Heather L.|last2=Levac|first2=Danielle|last3=O'Brien|first3=Kelly K.|last4=Straus|first4=Sharon|last5=Tricco|first5=Andrea C.|last6=Perrier|first6=Laure|last7=Kastner|first7=Monika|last8=Moher|first8=David|date=2014-12|title=Scoping reviews: time for clarity in definition, methods, and reporting|url=https://pubmed.ncbi.nlm.nih.gov/25034198/|journal=Journal of Clinical Epidemiology|volume=67|issue=12|pages=1291–1294|doi=10.1016/j.jclinepi.2014.03.013|issn=1878-5921|pmid=25034198}}</ref> This process is further complicated if it is mapping concepts across multiple languages or cultures.


As a scoping review should be systematically conducted and reported (with a transparent and repeatable method), some academic publishers categorize them as a kind of 'systematic review', which may cause confusion. Scoping reviews are helpful when it is not possible to carry out a systematic synthesis of research findings, for example, when there are no published clinical trials in the area of inquiry. Scoping reviews are helpful when determining if it is possible or appropriate to carry out a systematic review, and are a useful method when an area of inquiry is very broad,<ref>{{Cite journal|last=Lacaze|first=Paul|last2=Fransquet|first2=Peter|last3=Tiller|first3=Jane|last4=Nunn|first4=Jack S.|date=2019|title=Public Involvement in Global Genomics Research: A Scoping Review|url=https://www.frontiersin.org/articles/10.3389/fpubh.2019.00079/full|journal=Frontiers in Public Health|language=en|volume=7|doi=10.3389/fpubh.2019.00079|issn=2296-2565}}</ref> for example, exploring how the public are involved in all stages systematic reviews.<ref name="ReferenceA2">{{cite journal|vauthors=Pollock A, Campbell P, Struthers C, Synnot A, Nunn J, Hill S, Goodare H, Watts C, Morley R|date=2017-04-21|title=Stakeholder involvement in systematic reviews: a protocol for a systematic review of methods, outcomes and effects|journal=Research Involvement and Engagement|volume=3|issue=1|pages=9|doi=10.1186/s40900-017-0060-4|pmc=5611627|pmid=29062534}}</ref>
'''Scoping reviews'''<!--boldface per [[WP:R#PLA]]--> are distinct from systematic reviews in a number of important ways. A scoping review is an attempt to search for concepts, mapping the language which surrounds those and adjusting the search method iteratively.<ref>{{cite journal|doi=10.1080/1364557032000119616|title = Scoping studies: Towards a methodological framework|journal = International Journal of Social Research Methodology|volume = 8|pages = 19–32|year = 2005|last1 = Arksey|first1 = Hilary|last2 = O'Malley|first2 = Lisa |s2cid = 12719181| name-list-style = vanc |url = http://eprints.whiterose.ac.uk/1618/1/Scopingstudies.pdf}}</ref> A [[scoping review]] may often be a preliminary stage before a systematic review, which 'scopes' out an area of inquiry and maps the language and key concepts. As it is a kind of review which should be systematically conducted (the method is repeatable), some academic publishers categorize them as a kind of 'systematic review', which may cause confusion. Scoping reviews are helpful when it is not possible to carry out a systematic synthesis of research findings, for example, when there are no published clinical trials in the area of inquiry. Scoping reviews are a helpful method when an area of inquiry is very broad, for example, exploring how the public are involved in all stages systematic reviews.<ref name="ReferenceA">{{cite journal | vauthors = Pollock A, Campbell P, Struthers C, Synnot A, Nunn J, Hill S, Goodare H, Watts C, Morley R | title = Stakeholder involvement in systematic reviews: a protocol for a systematic review of methods, outcomes and effects | journal = Research Involvement and Engagement | volume = 3 | issue = 1 | pages = 9 | date = 2017-04-21 | pmid = 29062534 | pmc = 5611627 | doi = 10.1186/s40900-017-0060-4 }}</ref> There is still a lack of clarity when defining the exact method of scoping review as it is both an iterative process and is still relatively new and there have been a number of attempts to improve the standardisation of the method.<ref>{{cite journal | vauthors = Levac D, Colquhoun H, O'Brien KK | title = Scoping studies: advancing the methodology | journal = Implementation Science | volume = 5 | issue = 1 | pages = 69 | date = September 2010 | pmid = 20854677 | pmc = 2954944 | doi = 10.1186/1748-5908-5-69 }}</ref><ref>{{cite journal | vauthors = Peters MD, Godfrey CM, Khalil H, McInerney P, Parker D, Soares CB | title = Guidance for conducting systematic scoping reviews | journal = International Journal of Evidence-Based Healthcare | volume = 13 | issue = 3 | pages = 141–6 | date = September 2015 | pmid = 26134548 | doi = 10.1097/XEB.0000000000000050 | s2cid = 8860037 }}</ref><ref>{{cite journal | vauthors = Colquhoun HL, Levac D, O'Brien KK, Straus S, Tricco AC, Perrier L, Kastner M, Moher D | title = Scoping reviews: time for clarity in definition, methods, and reporting | journal = Journal of Clinical Epidemiology | volume = 67 | issue = 12 | pages = 1291–4 | date = December 2014 | pmid = 25034198 | doi = 10.1016/j.jclinepi.2014.03.013 | hdl = 1807/73365 | hdl-access = free }}</ref><ref>{{cite journal|last1=Arksey|first1=Hilary|last2=O'Malley|first2=Lisa | name-list-style = vanc |date=2005-02-01|title=Scoping studies: towards a methodological framework |journal=International Journal of Social Research Methodology|volume=8|issue=1|pages=19–32|doi=10.1080/1364557032000119616 |s2cid=12719181|url=http://eprints.whiterose.ac.uk/1618/1/Scopingstudies.pdf}}</ref><ref>{{Cite web|url=http://www.equator-network.org/reporting-guidelines/prisma-scr/|title=PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation {{!}} The EQUATOR Network|language=en-US|access-date=2019-11-12}}</ref> PROSPERO does not permit the submission of protocols of scoping reviews,<ref>{{cite web|url=https://www.crd.york.ac.uk/prospero/#aboutregpage |title= PROSPER O| work = Centre for Reviews and Dissemination | publisher = University of York |access-date=2019-02-24}}</ref> although some journals will publish protocols for scoping reviews.<ref name="ReferenceA"/>


There is still a lack of clarity when defining the exact method of a scoping review as it is both an iterative process and is still relatively new.<ref>{{Cite journal|last=Munn|first=Zachary|last2=Peters|first2=Micah D. J.|last3=Stern|first3=Cindy|last4=Tufanaru|first4=Catalin|last5=McArthur|first5=Alexa|last6=Aromataris|first6=Edoardo|date=2018-12|title=Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach|url=https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/s12874-018-0611-x|journal=BMC Medical Research Methodology|language=en|volume=18|issue=1|pages=143|doi=10.1186/s12874-018-0611-x|issn=1471-2288|pmc=PMC6245623|pmid=30453902}}</ref> There have been several attempts to improve the standardisation of the method,<ref>{{cite journal|vauthors=Levac D, Colquhoun H, O'Brien KK|date=September 2010|title=Scoping studies: advancing the methodology|journal=Implementation Science|volume=5|issue=1|pages=69|doi=10.1186/1748-5908-5-69|pmc=2954944|pmid=20854677}}</ref><ref>{{cite journal|vauthors=Peters MD, Godfrey CM, Khalil H, McInerney P, Parker D, Soares CB|date=September 2015|title=Guidance for conducting systematic scoping reviews|journal=International Journal of Evidence-Based Healthcare|volume=13|issue=3|pages=141–6|doi=10.1097/XEB.0000000000000050|pmid=26134548}}</ref><ref>{{cite journal|vauthors=Colquhoun HL, Levac D, O'Brien KK, Straus S, Tricco AC, Perrier L, Kastner M, Moher D|date=December 2014|title=Scoping reviews: time for clarity in definition, methods, and reporting|journal=Journal of Clinical Epidemiology|volume=67|issue=12|pages=1291–4|doi=10.1016/j.jclinepi.2014.03.013|pmid=25034198}}</ref><ref>{{cite journal|last=Arksey|first=Hilary|last2=O'Malley|first2=Lisa|date=2005-02-01|title=Scoping studies: towards a methodological framework|journal=International Journal of Social Research Methodology|volume=8|issue=1|pages=19–32|doi=10.1080/1364557032000119616|name-list-format=vanc}}</ref> for example via a PRISMA guideline extension for scoping reviews (PRISMA-ScR).<ref>{{Cite journal|last=Tricco|first=Andrea C.|last2=Lillie|first2=Erin|last3=Zarin|first3=Wasifa|last4=O'Brien|first4=Kelly K.|last5=Colquhoun|first5=Heather|last6=Levac|first6=Danielle|last7=Moher|first7=David|last8=Peters|first8=Micah D.J.|last9=Horsley|first9=Tanya|date=2018-10-02|title=PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation|url=http://annals.org/article.aspx?doi=10.7326/M18-0850|journal=Annals of Internal Medicine|language=en|volume=169|issue=7|pages=467|doi=10.7326/M18-0850|issn=0003-4819}}</ref> [[wikipedia:PROSPERO|PROSPERO]] (the International Prospective Register of Systematic Reviews) does not permit the submission of protocols of scoping reviews,<ref>{{cite web|title=PROSPERO|url=https://www.crd.york.ac.uk/prospero/#aboutregpage|access-date=2019-02-24|work=Centre for Reviews and Dissemination|publisher=University of York}}</ref> although some journals will publish protocols for scoping reviews.<ref name="ReferenceA2" />
== Stages ==
The main stages of a systematic review are:


{{Fig|number=1|image=Extraction machine.gif|caption=A visualisation of data being 'extracted' and 'combined' in a Cochrane intervention effect review where a meta-analysis is possible.<ref name="Cochrane storyboard" />|attribution=|align=right|size=440px}}While there are multiple kinds of systematic review methods, the main stages of a review can be summarised into five stages:
# ''Defining a question'' and agreeing an objective method.<ref name="Cochrane storyboard">{{cite web|url=https://cccrg.cochrane.org/animated-storyboard-what-are-systematic-reviews|title=Animated Storyboard: What Are Systematic Reviews?|website=cccrg.cochrane.org|publisher=[[Cochrane (organisation)|Cochrane Consumers and Communication]]|access-date=1 June 2016}}</ref> It is considered best practice to publish the protocol of the systematic review before initiating it in order to help avoid unplanned duplication and to enable comparison of reported review methods with what was planned in the protocol.<ref>{{cite web |url= http://www.prisma-statement.org/Protocols/Registration |title=PRISMA|work = Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) |access-date=2019-02-24}}</ref>
# ''A search for relevant data'' from research that matches certain criteria. For example, only selecting research that is good quality and answers the defined question.<ref name="Cochrane storyboard"/> Contacting a trained information professional or librarian can improve the quality of the systematic review.<ref>{{cite journal | vauthors = Rethlefsen ML, Farrell AM, Osterhaus Trzasko LC, Brigham TJ | title = Librarian co-authors correlated with higher quality reported search strategies in general internal medicine systematic reviews | journal = Journal of Clinical Epidemiology | volume = 68 | issue = 6 | pages = 617–26 | date = June 2015 | pmid = 25766056 | doi = 10.1016/j.jclinepi.2014.11.025 }}</ref>
# ''<nowiki/>'Extraction' of relevant data.'' This can include how the research was done (often called the method or 'intervention'), who participated in the research (including how many people), how it was paid for (for example funding sources) and what happened (the outcomes).<ref name="Cochrane storyboard"/>
# ''Assess the quality of the data'' by judging it against criteria identified at the first stage.<ref name="Cochrane storyboard"/> This can include assessing the quality (or certainty) of evidence, using criteria such as GRADE.<ref>{{Cite web|url=http://www.gradeworkinggroup.org/|title=GRADE home|website=www.gradeworkinggroup.org|access-date=2019-03-08}}</ref>
# ''Analyse and combine the data'' (using complex statistical methods) which give an overall result from all of the data. This combination of data can be visualised using a [[blobbogram]] (also called a [[forest plot]]).<ref name="Cochrane storyboard"/> The diamond in the blobbogram represents the combined results of all the data included. Because this combined result uses data from more sources than just one data set, it's considered more reliable and better evidence, as the more data there is, the more confident we can be of conclusions.<ref name="Cochrane storyboard"/>
Once these stages are complete, the review may be published, disseminated and translated into practice after being adopted as evidence.


=== Living systematic reviews ===
=== 1. Defining the research question ===
Defining an answerable question and agreeing an objective method is required to design a useful systematic review.<ref name="Cochrane storyboard2">{{cite web|title=Animated Storyboard: What Are Systematic Reviews?|url=https://cccrg.cochrane.org/animated-storyboard-what-are-systematic-reviews|access-date=1 June 2016|website=cccrg.cochrane.org|publisher=Cochrane Consumers and Communication}}</ref> Best practice recommends publishing the protocol of the review before initiating it to reduce the risk of unplanned research duplication and to enable consistency between methodology and protocol.<ref>{{cite web|title=PRISMA|url=http://www.prisma-statement.org/Protocols/Registration|access-date=2019-02-24|work=Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)}}</ref> Clinical reviews of quantitative data are often structured using the acronym [[PICO process|PICO]], which stands for 'Population or Problem', 'Intervention or Exposure', 'Comparison' and 'Outcome', with other variations existing for other kinds of research. For qualitative reviews PICo is 'Population or Problem', 'Interest' and 'Context'.
Living systematic reviews are a relatively new kind of high quality, semi-automated, up-to-date online summaries of research which are updated as new research becomes available.<ref>{{cite journal | vauthors = Tsafnat G, Glasziou P, Choong MK, Dunn A, Galgani F, Coiera E | title = Systematic review automation technologies | journal = Systematic Reviews | volume = 3 | issue = 1 | pages = 74 | date = July 2014 | pmid = 25005128 | pmc = 4100748 | doi = 10.1186/2046-4053-3-74 }}</ref> The essential difference between 'living systematic review' and conventional systematic review is the publication format. Living systematic reviews are 'dynamic, persistent, online-only evidence summaries, which are updated rapidly and frequently'.<ref>{{cite journal | vauthors = Elliott JH, Turner T, Clavisi O, Thomas J, Higgins JP, Mavergames C, Gruen RL | title = Living systematic reviews: an emerging opportunity to narrow the evidence-practice gap | journal = PLOS Medicine | volume = 11 | issue = 2 | pages = e1001603 | date = February 2014 | pmid = 24558353 | pmc = 3928029 | doi = 10.1371/journal.pmed.1001603 }}</ref>

=== 2. Searching for relevant data sources ===
Planning how the review will search for relevant data from research that matches certain criteria is a decisive stage in developing a rigorous systematic review. Relevant criteria can include only selecting research that is good quality and answers the defined question.<ref name="Cochrane storyboard2" /> The search strategy should be designed to retrieve literature that matches the protocol's specified inclusion and exclusion criteria.

The methodology section of a systematic review should list all of the databases and citation indices that were searched. The titles and abstracts of identified articles can be checked against pre-determined criteria for eligibility and relevance. Each included study may be assigned an objective assessment of methodological quality, preferably by using methods conforming to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement,<ref name=":5" /> or the high-quality standards of Cochrane.<ref name="CochraneHandbook2">{{cite web|title=Cochrane handbook for systematic reviews of interventions, version 5.1.0 (updated March 2011)|url=http://handbook.cochrane.org|access-date=2 June 2016|publisher=The Cochrane Collaboration|veditors=Higgins JP, Green S}}</ref>

Common information sources used in searches include scholarly databases of peer-reviewed articles such as [[MEDLINE]], [[Web of Science]], [[Embase]], and [[PubMed]] as well as sources of unpublished literature such as clinical trial registries and [[grey literature]] collections. Key references can also be yielded through additional methods such as citation searching, reference list checking (related to a search method called '[[wikipedia:Pearl_growing|pearl growing]]'), manually searching information sources not indexed in the major electronic databases (sometimes called 'hand-searching'),<ref>{{Cite web|title=5. Handsearching|url=https://training.cochrane.org/resource/tsc-induction-mentoring-training-guide/5-handsearching|access-date=2020-09-14|website=training.cochrane.org|language=en}}</ref> and directly contacting experts in the field.<ref>{{Cite journal|last=Papaioannou|first=Diana|last2=Sutton|first2=Anthea|last3=Carroll|first3=Christopher|last4=Booth|first4=Andrew|last5=Wong|first5=Ruth|date=2010|title=Literature searching for social science systematic reviews: consideration of a range of search techniques|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1471-1842.2009.00863.x|journal=Health Information & Libraries Journal|language=en|volume=27|issue=2|pages=114–122|doi=10.1111/j.1471-1842.2009.00863.x|issn=1471-1842}}</ref>

To be systematic, searchers must use a combination of search skills and tools such as database subject headings, keyword searching, [[Logical connective|Boolean operators]], proximity searching, while attempting to balance the sensitivity (systematicity) and precision (accuracy). Inviting and involving an experienced information professional or librarian can notably improve the quality of systematic review search strategies and reporting.<ref>{{cite journal|vauthors=Rethlefsen ML, Farrell AM, Osterhaus Trzasko LC, Brigham TJ|date=June 2015|title=Librarian co-authors correlated with higher quality reported search strategies in general internal medicine systematic reviews|journal=Journal of Clinical Epidemiology|volume=68|issue=6|pages=617–26|doi=10.1016/j.jclinepi.2014.11.025|pmid=25766056}}</ref><ref>{{Cite journal|last=Koffel|first=Jonathan B.|date=2015-05-04|title=Use of Recommended Search Strategies in Systematic Reviews and the Impact of Librarian Involvement: A Cross-Sectional Survey of Recent Authors|url=https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0125931|journal=PLOS ONE|language=en|volume=10|issue=5|pages=e0125931|doi=10.1371/journal.pone.0125931|issn=1932-6203|pmc=PMC4418838|pmid=25938454}}</ref><ref>{{Cite journal|last=Costella|first=John|last2=Torabi|first2=Nazi|last3=Meert|first3=Deborah|date=2016|title=Impact of librarians on reporting of the literature searching component of pediatric systematic reviews|url=http://jmla.pitt.edu/ojs/jmla/article/view/139|journal=Journal of the Medical Library Association|language=en|volume=104|issue=4|pages=267–277|doi=10.5195/jmla.2016.139|issn=1558-9439|pmc=PMC5079487|pmid=27822147}}</ref><ref>{{Cite journal|last=Yang|first=Kehu|last2=Yao|first2=Liang|last3=Jiang|first3=Tongxiao|last4=Liang|first4=Fuxiang|last5=Moher|first5=David|last6=Tian|first6=Hongliang|last7=Tian|first7=Jinhui|last8=Li|first8=Lun|date=2014-09-01|title=Network meta-analyses could be improved by searching more sources and by involving a librarian|url=https://www.jclinepi.com/article/S0895-4356(14)00116-4/abstract|journal=Journal of Clinical Epidemiology|language=en|volume=67|issue=9|pages=1001–1007|doi=10.1016/j.jclinepi.2014.04.003|issn=0895-4356|pmid=24841794}}</ref><ref>{{Cite journal|last=Rethlefsen|first=Melissa L.|last2=Murad|first2=M. Hassan|last3=Livingston|first3=Edward H.|date=2014-09-10|title=Engaging Medical Librarians to Improve the Quality of Review Articles|url=http://jama.jamanetwork.com/article.aspx?doi=10.1001/jama.2014.9263|journal=JAMA|language=en|volume=312|issue=10|pages=999|doi=10.1001/jama.2014.9263|issn=0098-7484}}</ref>

=== 3. 'Extraction' of relevant data ===
Relevant data are 'extracted' from the data sources according to the review method. It is important to note that the data extraction method is specific to the kind of data, and data extracted on ‘outcomes’ is only relevant to certain types of reviews. For example, a systematic review of clinical trials might extract data about how the research was done (often called the method or 'intervention'), who participated in the research (including how many people), how it was paid for (for example funding sources) and what happened (the outcomes).<ref name="Cochrane storyboard2" /> '''Figure 1''' illustrates relevant data being extracted and 'combined' in a Cochrane intervention effect review, where a meta-analysis is possible.

=== 4. Assess the eligibility of the data ===
This stage involves assessing the eligibility of data for inclusion in the review, by judging it against criteria identified at the first stage.<ref name="Cochrane storyboard2" /> This can include assessing if a data source meets the eligibility criteria, and recording why decisions about inclusion or exclusion in the review were made. Software can be used to support the selection process including text mining tools and machine learning, which can automate aspects of the process.<ref>{{Cite book|last=|first=|url=https://training.cochrane.org/handbook/current|title=Cochrane Handbook for Systematic Reviews of Interventions|date=2019-09-20|website=training.cochrane.org|publisher=|year=|isbn=|editor-last=Higgins|editor-first=Julian P.T.|series=version 6.1|location=|pages=section 4.6|nopp=y|language=en|chapter=Chapter 4: Searching for and selecting studies|access-date=2020-09-14|editor2-last=Thomas|editor2-first=James|editor3-last=Chandler|editor3-first=Jacqueline|editor4-last=Cumpston|editor4-first=Miranda|editor5-last=Li|editor5-first=Tianjing|editor6-last=Page|editor6-first=Matthew J.|editor7-last=Welch|editor7-first=Vivian A.|chapter-url=https://training.cochrane.org/handbook/current/chapter-04#section-4-6}}</ref> The ‘Systematic Review Toolbox’ is a community driven, web-based catalogue of tools, to help reviewers chose appropriate tools for reviews.<ref>{{Cite journal|last=Marshall|first=Christopher|last2=Brereton|first2=Pearl|date=2015-04-27|title=Systematic review toolbox: a catalogue of tools to support systematic reviews|url=https://doi.org/10.1145/2745802.2745824|journal=Proceedings of the 19th International Conference on Evaluation and Assessment in Software Engineering|series=EASE '15|location=Nanjing, China|publisher=Association for Computing Machinery|pages=1–6|doi=10.1145/2745802.2745824|isbn=978-1-4503-3350-4}}</ref>

=== 5. Analyse and combine the data ===
Analysing and combining data can provide an overall result from all the data. Because this combined result uses qualitative or quantitative data from all eligible sources of data, it is considered more reliable as it provides better evidence, as the more data included in reviews, the more confident we can be of conclusions. When appropriate, some systematic reviews include a meta-analysis, which uses statistical methods to combine data from multiple sources. A review might use quantitative data, or might employ a qualitative meta-synthesis, which synthesises data from qualitative studies. The combination of data from a meta-analysis can sometimes be visualised. One method uses a a [[forest plot]] (also called a [[blobbogram]]).<ref name="Cochrane storyboard2" /> In an intervention effect review, the diamond in the 'forest plot' represents the combined results of all the data included.<ref name="Cochrane storyboard2" />

An example of a 'forest plot' is the Cochrane Collaboration logo.<ref name="Cochrane storyboard2" /> The logo is a forest plot of one of the first reviews which showed that corticosteroids given to women who are about to give birth prematurely can save the life of the newborn child.<ref>{{Cite web|title=The difference we make|url=https://www.cochrane.org/about-us/difference-we-make|access-date=2019-03-08|website=www.cochrane.org}}</ref>

Recent visualisation innovations include the albatross plot, which plots p-values against sample sizes, with approximate effect-size contours superimposed to facilitate analysis.<ref>{{Cite journal|last=Higgins|first=Julian P. T.|last2=López-López|first2=José A.|last3=Becker|first3=Betsy J.|last4=Davies|first4=Sarah R.|last5=Dawson|first5=Sarah|last6=Grimshaw|first6=Jeremy M.|last7=McGuinness|first7=Luke A.|last8=Moore|first8=Theresa H. M.|last9=Rehfuess|first9=Eva A.|date=2019-01-01|title=Synthesising quantitative evidence in systematic reviews of complex health interventions|url=https://gh.bmj.com/content/4/Suppl_1/e000858|journal=BMJ Global Health|language=en|volume=4|issue=Suppl 1|pages=e000858|doi=10.1136/bmjgh-2018-000858|issn=2059-7908}}</ref> The contours can be used to infer effect sizes from studies that have been analysed and reported in diverse ways. Such visualisations may have advantages over other types when reviewing complex interventions.

Assessing the quality (or certainty) of evidence is an important part of some reviews. GRADE (Grading of Recommendations, Assessment, Development and Evaluations) is a transparent framework for developing and presenting summaries of evidence and is used to grade the quality of evidence.<ref>{{Cite web|title=GRADE working group|url=http://www.gradeworkinggroup.org/|access-date=2019-03-08|website=www.gradeworkinggroup.org}}</ref> The GRADE-CERQual (Confidence in the Evidence from Reviews of Qualitative research) is used to provide a transparent method for assessing the confidence of evidence from reviews or qualitative research.<ref>{{Cite web|title=GRADE CERQual|url=https://www.cerqual.org/|access-date=2020-07-01|website=www.cerqual.org|language=en}}</ref> Once these stages are complete, the review may be published, disseminated and translated into practice after being adopted as evidence.

== Living systematic reviews ==
Living systematic reviews are a relatively new kind of high quality, semi-automated, up-to-date online summaries of research which are updated as new research becomes available.<ref>{{cite journal|vauthors=Tsafnat G, Glasziou P, Choong MK, Dunn A, Galgani F, Coiera E|date=July 2014|title=Systematic review automation technologies|journal=Systematic Reviews|volume=3|issue=1|pages=74|doi=10.1186/2046-4053-3-74|pmc=4100748|pmid=25005128}}</ref> The essential difference between a living systematic review and a conventional systematic review is the publication format. Living systematic reviews are 'dynamic, persistent, online-only evidence summaries, which are updated rapidly and frequently'.<ref>{{cite journal|vauthors=Elliott JH, Turner T, Clavisi O, Thomas J, Higgins JP, Mavergames C, Gruen RL|date=February 2014|title=Living systematic reviews: an emerging opportunity to narrow the evidence-practice gap|journal=PLoS Medicine|volume=11|issue=2|pages=e1001603|doi=10.1371/journal.pmed.1001603|pmc=3928029|pmid=24558353}}</ref>


==Research fields==
==Research fields==
===Medicine and biology===


==== History ====
=== Medicine and human health ===


==== History of systematic reviews in medicine ====
In 1753, [[James Lind]] published ''A Treatise on the Scurvy'', a study which he described in the subtitle as ''A Critical and Chronological View of What has been Published on the Subject''. This is the first which can be said as systematic review.<ref>{{cite journal |last1=Bartholomew |first1=M |title=James Lind's Treatise of the Scurvy (1753) |journal=Postgraduate Medical Journal |date=2002 |volume=78 |issue=925 |pages=695–696 |doi=10.1136/pmj.78.925.695 |pmid=12496338 |pmc=1742547}}</ref> Lind comments, "It became requisite to exhibit a full and impartial view of what had hitherto been published on the scurvy," and, "before the subject could be set in a clear and proper light, it was necessary to remove a great deal of rubbish."<ref name="clarke18">{{cite journal |last1=Clarke |first1=Mike |last2=Chalmers |first2=Iain |title=Reflections on the history of systematic reviews |journal=BMJ Evidence-Based Medicine |date=2018 |volume=23 |issue=4 |pages=121–122 |doi=10.1136/bmjebm-2018-110968 |pmid=29921711|doi-access=free }}</ref> The first proper analysis of different sources was a report made by [[Karl Pearson]] in 1904.<ref name="gurevitch18">{{cite journal |last1=Gurevitch |first1=Jessica |last2=Koricheva |first2=Julia |last3=Nakagawa |first3=Shinichi |last4=Stewart |first4=Gavin |title=Meta-analysis and the science of research synthesis |journal=Nature |date=2018 |volume=555 |issue=7695 |pages=175–182 |doi=10.1038/nature25753 |pmid=29517004|bibcode=2018Natur.555..175G |s2cid=3761687 }}</ref> Pearson analysed the effectiveness of [[typhoid vaccine]] from five studies of immunity and six studies of mortality. What amazed him was the irregularity of the data. He wrote:
A 1904 ''British Medical Journal'' paper by [[wikipedia:Karl_Pearson|Karl Pearson]] collated data from several studies in the UK, India and South Africa of typhoid inoculation. He used a meta-analytic approach to aggregate the outcomes of multiple clinical studies.<ref>{{Cite journal|last=Group|first=British Medical Journal Publishing|date=1904-11-05|title=Report on Certain Enteric Fever Inoculation Statistics|url=https://www.bmj.com/content/2/2288/1243|journal=BMJ|language=en|volume=2|issue=2288|pages=1243–1246|doi=10.1136/bmj.2.2288.1243|issn=0007-1447|via=}}</ref> In 1972 Archie Cochrane wrote: 'It is surely a great criticism of our profession that we have not organised a critical summary, by specialty or subspecialty, adapted periodically, of all relevant randomised controlled trials'.<ref name=":7">{{cite web|last=|first=|date=|title=1.1.2 A brief history of Cochrane|url=https://community.cochrane.org/handbook-sri/chapter-1-introduction/11-cochrane/112-brief-history-cochrane|archive-url=https://web.archive.org/web/20190117070242/https://community.cochrane.org/handbook-sri/chapter-1-introduction/11-cochrane/112-brief-history-cochrane|archive-date=2019-01-17|access-date=2019-02-24|website=community.cochrane.org|language=en}}</ref> Critical appraisal and synthesis of research findings in a systematic way emerged in 1975 under the term 'meta analysis'.<ref>{{cite book|last=Glass|first=Gene V.|url=https://catalogue.nla.gov.au/Record/5363514|title=Meta-Analysis of Research on the Relationship of Class-Size and Achievement. The Class Size and Instruction Project|last2=Smith|first2=Mary Lee|collaboration=Far West Lab. for Educational Research and Development, San Francisco, CA|date=1978|publisher=Distributed by ERIC Clearinghouse|location=Washington, D.C.]|name-list-format=vanc}}</ref><ref>{{cite web|title=History of Systematic Reviews|url=https://eppi.ioe.ac.uk/cms/Resources/EvidenceInformedPolicyandPractice/HistoryofSystematicReviews/tabid/68/Default.aspx|access-date=2019-02-24|work=Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre)}}</ref> Early syntheses were conducted in broad areas of public policy and social interventions, with systematic research synthesis applied to medicine and health.<ref>{{Cite journal|last=Lau|first=J.|last2=Antman|first2=E. M.|last3=Jimenez-Silva|first3=J.|last4=Kupelnick|first4=B.|last5=Mosteller|first5=F.|last6=Chalmers|first6=T. C.|date=1992-07-23|title=Cumulative meta-analysis of therapeutic trials for myocardial infarction|url=https://pubmed.ncbi.nlm.nih.gov/1614465/|journal=The New England Journal of Medicine|volume=327|issue=4|pages=248–254|doi=10.1056/NEJM199207233270406|issn=0028-4793|pmid=1614465}}</ref> Inspired by his own personal experiences as a senior medical officer in prisoner of war camps, [[wikipedia:Archie_Cochrane|Archie Cochrane]] worked to improve how the scientific method was used in medical evidence, writing in 1971: 'the general scientific problem with which we are primarily concerned is that of testing a hypothesis that a certain treatment alters the natural history of a disease for the better'.<ref>{{Cite book|last=Cochrane, A. L.|first=|url=https://www.worldcat.org/oclc/741462|title=Effectiveness and efficiency: random reflections on health services|date=1972|publisher=Nuffield Provincial Hospitals Trust|year=|isbn=0-900574-17-8|location=[London]|pages=|oclc=741462|archive-url=https://web.archive.org/web/20200916053459/https://www.nuffieldtrust.org.uk/files/2017-01/effectiveness-and-efficiency-web-final.pdf|archive-date=2020-09-16}}</ref> His call for the increased use of randomised controlled trials and systematic reviews led to the creation of The Cochrane Collaboration,<ref>{{Cite journal|last=Shah|first=Hriday M.|last2=Chung|first2=Kevin C.|date=2009-09|title=Archie Cochrane and his vision for evidence-based medicine|url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746659/|journal=Plastic and reconstructive surgery|language=en|volume=124|issue=3|pages=982|doi=10.1097/PRS.0b013e3181b03928|pmid=19730323}}</ref> which was founded in 1993 and named after him, building on the work by [[Iain Chalmers]] and colleagues in the area of pregnancy and childbirth.<ref>{{Cite book|last=Evans, Imogen.|url=https://www.worldcat.org/oclc/778837501|title=Testing treatments : better research for better healthcare|date=2011|publisher=Pinter & Martin|others=Thornton, Hazel, 1935-, Chalmers, Iain., Glasziou, Paul, 1954-|isbn=978-1-905177-48-6|edition=Second edition|location=London|oclc=778837501}}</ref><ref name=":7" />


==== Current use of systematic reviews in medicine ====
{{Quote|Assuming that the inoculation is not more than a temporary inconvenience, it would seem to be possible to call for volunteers... [and] only to inoculate every second volunteer... with a view to ascertaining whether any inoculation is likely to prove useful... In other words, the 'experiment' might demonstrate that this first step to a reasonably effective prevention was not a false one.<ref>{{cite journal |last1=Pearson |first1=K |title=Report on Certain Enteric Fever Inoculation Statistics |journal=BMJ |date=1904 |volume=2 |issue=2288 |pages=1243–1246 |doi=10.1136/bmj.2.2288.1243 |pmid=20761760 |pmc=2355479}}</ref>}}
Many organisations around the world use systematic reviews, with the methodology depending on the guidelines being followed. Organisations which use systematic reviews in medicine and human health include the [[wikipedia:National_Institute_for_Health_and_Care_Excellence|National Institute for Health and Care Excellence]] (NICE, UK), the [[wikipedia:Agency_for_Healthcare_Research_and_Quality|Agency for Healthcare Research and Quality]] (AHRQ, USA) and the [[wikipedia:World_Health_Organization|World Health Organisation]]. Most notable among international organisations is [[Cochrane Collaboration|Cochrane]], a group of over 37,000 specialists in healthcare who systematically review randomised trials of the effects of prevention, treatments and rehabilitation as well as health systems interventions. When appropriate, they also include the results of other types of research. Cochrane Reviews are published in ''The Cochrane Database of Systematic Reviews'' section of the [[Cochrane Library]]. The 2015 [[impact factor]] for ''The Cochrane Database of Systematic Reviews'' was 6.103, and it was ranked 12th in the Medicine, General & Internal category.<ref>The Cochrane Library. [http://www.cochrane.org/news/2015-impact-factor-released-cochrane-database-systematic-reviews 2015 impact factor. Cochrane Database of Systematic Reviews (CDSR)] Retrieved 2016-07-20.</ref>


There are several types of Cochrane Review, including:<ref>[http://tech.cochrane.org/revman <nowiki>Review Manager (RevMan) [Computer program]</nowiki>]. Version 5.2. Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2012.</ref><ref>{{Cite web|last=|first=|date=|title=Main page|url=https://www.cochranelibrary.com/|archive-url=|archive-date=|access-date=|website=Cochrane Library|language=en}}</ref><ref>{{cite journal|last=|first=|vauthors=Silva V, Grande AJ, Carvalho AP, Martimbianco AL, Riera R|date=|year=2015|title=Overview of systematic reviews - a new type of study. Part II|url=|journal=Revista Paulista de Medicina|volume=133|issue=3|pages=206–17|doi=10.1590/1516-3180.2013.8150015|pmid=25388685|via=}}</ref><ref>{{cite journal|last=|first=|vauthors=Silva V, Grande AJ, Martimbianco AL, Riera R, Carvalho AP|date=|year=2012|title=Overview of systematic reviews - a new type of study: part I: why and for whom?|url=|journal=Revista Paulista de Medicina|volume=130|issue=6|pages=398–404|doi=10.1590/S1516-31802012000600007|pmid=23338737|via=}}</ref>
This is considered as the first analysis of clinical trials using what is later known as meta-analysis.<ref name=orourke>{{cite journal |last1=O'Rourke |first1=K. |title=An historical perspective on meta-analysis: dealing quantitatively with varying study results |journal=Journal of the Royal Society of Medicine |date=2007 |volume=100 |issue=12 |pages=579–582 |doi=10.1258/jrsm.100.12.579 |pmid=18065712 |pmc=2121629}}</ref>


# '''Intervention reviews''' assess the benefits and harms of interventions used in healthcare and health policy.
In 1925, [[Ronald Fisher]] introduced the statistical method ([[probability]]) for such analysis in his book ''Statistical Methods for Research Workers''.<ref name="gurevitch18"/> A colleague of Fisher, [[William Gemmell Cochran]] made an improved method in 1937.<ref>{{cite journal |last1=Cochran |first1=W. G. |title=Problems Arising in the Analysis of a Series of Similar Experiments |journal=Supplement to the Journal of the Royal Statistical Society |date=1937 |volume=4 |issue=1 |pages=102–118 |doi=10.2307/2984123 |jstor=2984123}}</ref> With [[Frank Yates]], Cochran reported analysis of agricultural data in 1938 titled "The analysis of groups of experiments" using the new method.<ref>{{cite journal |last1=Yates |first1=F. |last2=Cochran |first2=W. G. |title=The analysis of groups of experiments |journal=The Journal of Agricultural Science |date=1938 |volume=28 |issue=4 |pages=556–580 |doi=10.1017/S0021859600050978}}</ref>
# '''Diagnostic test accuracy reviews''' assess how well a diagnostic test performs in diagnosing and detecting a particular disease.
# '''Methodology reviews''' address issues relevant to how systematic reviews and clinical trials are conducted and reported.
# '''Qualitative reviews''' synthesize qualitative evidence to address questions on aspects other than effectiveness.
# '''Prognosis reviews''' address the probable course or future outcome(s) of people with a health problem.
# '''Overviews of Systematic Reviews (OoRs)''' are a new type of study to compile multiple evidence from systematic reviews into a single document that is accessible and useful to serve as a friendly front end for the Cochrane Collaboration with regard to healthcare decision-making. These are sometimes referred to as 'umbrella reviews'.
# '''Living Systematic reviews''' are continually updated, incorporating relevant new evidence as it becomes available.<ref>{{Cite web|title=Living systematic reviews|url=https://community.cochrane.org/review-production/production-resources/living-systematic-reviews|access-date=2020-07-01|website=community.cochrane.org|language=en}}</ref> They are a relatively new kind of review, with methods still being developed and evaluated. They can be high quality, semi-automated, up-to-date online summaries of research which are updated as new research becomes available.<ref>{{Cite journal|last=Tsafnat|first=Guy|last2=Glasziou|first2=Paul|last3=Choong|first3=Miew Keen|last4=Dunn|first4=Adam|last5=Galgani|first5=Filippo|last6=Coiera|first6=Enrico|date=2014-12|title=Systematic review automation technologies|url=https://systematicreviewsjournal.biomedcentral.com/articles/10.1186/2046-4053-3-74|journal=Systematic Reviews|language=en|volume=3|issue=1|pages=74|doi=10.1186/2046-4053-3-74|issn=2046-4053|pmc=PMC4100748|pmid=25005128}}</ref> The essential difference between a 'living systematic review' and a conventional systematic review is the publication format. Living systematic reviews are 'dynamic, persistent, online-only evidence summaries, which are updated rapidly and frequently'.<ref>{{Cite journal|last=Elliott|first=Julian H.|last2=Turner|first2=Tari|last3=Clavisi|first3=Ornella|last4=Thomas|first4=James|last5=Higgins|first5=Julian P. T.|last6=Mavergames|first6=Chris|last7=Gruen|first7=Russell L.|date=2014-02-18|title=Living Systematic Reviews: An Emerging Opportunity to Narrow the Evidence-Practice Gap|url=https://dx.plos.org/10.1371/journal.pmed.1001603|journal=PLoS Medicine|language=en|volume=11|issue=2|pages=e1001603|doi=10.1371/journal.pmed.1001603|issn=1549-1676|pmc=PMC3928029|pmid=24558353}}</ref>
# '''Rapid reviews''' are a form of knowledge synthesis that ‘accelerates the process of conducting a traditional systematic review through streamlining or omitting specific methods to produce evidence for stakeholders in a resource-efficient manner’.<ref>{{Cite web|title=Cochrane&#039;s work on Rapid Reviews in response to COVID-19|url=https://www.cochrane.org/cochranes-work-rapid-reviews-response-covid-19|access-date=2020-07-01|website=www.cochrane.org|language=en}}</ref>
# '''Reviews of complex health interventions in complex systems''' review interventions and interventions delivered in complex systems to improve evidence synthesis and guideline development at a global, national or health systems level.<ref>{{Cite journal|last=Norris|first=Susan L.|last2=Rehfuess|first2=Eva A.|last3=Smith|first3=Helen|last4=Tunçalp|first4=Özge|last5=Grimshaw|first5=Jeremy M.|last6=Ford|first6=Nathan P.|last7=Portela|first7=Anayda|date=2019-01-01|title=Complex health interventions in complex systems: improving the process and methods for evidence-informed health decisions|url=https://gh.bmj.com/content/4/Suppl_1/e000963|journal=BMJ Global Health|language=en|volume=4|issue=Suppl 1|pages=e000963|doi=10.1136/bmjgh-2018-000963|issn=2059-7908}}</ref>


The Cochrane Collaboration provides a handbook for systematic reviewers of interventions which 'provides guidance to authors for the preparation of Cochrane Intervention reviews.'<ref name="CochraneHandbook2" /> The ''Cochrane Handbook'' also outlines the key steps for preparing a systematic review<ref name="CochraneHandbook2" /> and forms the basis of two sets of standards for the conduct and reporting of Cochrane Intervention Reviews (MECIR - Methodological Expectations of Cochrane Intervention Reviews).<ref>{{cite web|title=Methodological Expectations of Cochrane Intervention Reviews (MECIR)|url=http://www.editorial-unit.cochrane.org/mecir|access-date=6 October 2014|publisher=Cochrane}}</ref> It also contains guidance on how to undertake qualitative evidence synthesis, economic reviews and integrating patient-reported outcomes into reviews.
In 1974, [[Archie Cochrane]] and colleagues made analysis of the use of [[aspirin]] in the prevention of heart attack ([[myocardial infarction]]).<ref>{{cite journal |last1=Elwood |first1=P. C. |last2=Cochrane |first2=A. L. |last3=Burr |first3=M. L. |last4=Sweetnam |first4=P. M. |last5=Williams |first5=G. |last6=Welsby |first6=E. |last7=Hughes |first7=S. J. |last8=Renton |first8=R. |title=A Randomized Controlled Trial of Acetyl Salicyclic Acid in the Secondary Prevention of Mortality from Myocardial Infarction |journal=BMJ |date=1974 |volume=1 |issue=5905 |pages=436–440 |doi=10.1136/bmj.1.5905.436 |pmid=4593555 |pmc=1633246}}</ref> This became one of the most influential systematic analysis in [[randomized controlled trial]].<ref name=orourke/> In 1979, Archie Cochrane wrote 'It is surely a great criticism of our profession that we have not organised a critical summary, by specialty or subspecialty, adapted periodically, of all relevant randomised controlled trials'.<ref>{{cite web|url=https://community.cochrane.org/handbook-sri/chapter-1-introduction/11-cochrane/112-brief-history-cochrane|title=1.1.2 A brief history of Cochrane|website=community.cochrane.org|access-date=2019-02-24}}</ref> Critical appraisal and synthesis of research findings in a systematic way emerged in the 1970s after [[Gene V. Glass]] introduced the term 'meta analysis' in 1976. According to Glass, meta-analysis is "the statistical analysis of a large collection of analysis results from individual studies for the purpose of integrating the findings".<ref>{{cite journal |last1=Glass |first1=Gene V |title=Primary, Secondary, and Meta-Analysis of Research |journal=Educational Researcher |date=1976 |volume=5 |issue=10 |pages=3–8 |doi=10.3102/0013189X005010003|s2cid=3185455 }}</ref><ref>{{cite book|url=https://catalogue.nla.gov.au/Record/5363514|title=Meta-Analysis of Research on the Relationship of Class-Size and Achievement. The Class Size and Instruction Project|last1=Glass|first1=Gene V.|last2=Smith|first2=Mary Lee | name-list-style = vanc |date=1978|publisher=Distributed by ERIC Clearinghouse| collaboration = Far West Lab. for Educational Research and Development, San Francisco, CA|location=Washington, D.C.}}</ref><ref>{{cite web |url= https://eppi.ioe.ac.uk/cms/Resources/EvidenceInformedPolicyandPractice/HistoryofSystematicReviews/tabid/68/Default.aspx |title=History of Systematic Reviews| work = Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre) |access-date=2019-02-24}}</ref> Early syntheses were conducted in broad areas of public policy and social interventions, with systematic research synthesis applied to medicine and health. The Cochrane Collaboration was founded in 1993, building on the work by [[Iain Chalmers]] and colleagues in the area of pregnancy and childbirth.<ref>{{cite web |url= https://community.cochrane.org/handbook-sri/chapter-1-introduction/11-cochrane/112-brief-history-cochrane |title=1.1.2 A brief history of Cochrane |website=community.cochrane.org |access-date=2019-02-24 }}</ref>


The Cochrane Library is a collection of databases that contains different types of independent evidence to inform healthcare decision-making. It contains a database of systematic review and meta-analyses which summarize and interpret the results of multi-disciplinary research. The library contains the Cochrane Database of Systematic Reviews (CDSR), which is a journal and database for systematic reviews in health care. The Cochrane Library also contains the Cochrane Central Register of Controlled Trials (CENTRAL) which is a database of reports of randomized and quasi-randomized controlled trials.<ref>{{Cite web|last=|first=|date=|title=Cochrane Controlled Register of Trials (CENTRAL)|url=https://www.cochranelibrary.com/central/about-central|archive-url=|archive-date=|access-date=2020-07-01|website=Cochrane Library|language=en}}</ref> The Cochrane Library is also available in Spanish.<ref>{{Cite web|title=Revisiones Cochrane|url=https://www.cochranelibrary.com/es/home|access-date=2020-07-01|website=Cochrane Library|language=es}}</ref>
==== Contemporary ====
Named after Archie Cochrane, [[Cochrane Collaboration|Cochrane]] is a group of over 37,000 specialists in healthcare who systematically review randomised trials of the effects of prevention, treatments and rehabilitation as well as health systems interventions. When appropriate, they also include the results of other types of research. Cochrane Reviews are published in ''The Cochrane Database of Systematic Reviews'' section of the [[Cochrane Library]]. The 2015 [[impact factor]] for ''The Cochrane Database of Systematic Reviews'' was 6.103, and it was ranked 12th in the “Medicine, General & Internal” category.<ref>The Cochrane Library. [http://www.cochrane.org/news/2015-impact-factor-released-cochrane-database-systematic-reviews 2015 impact factor. Cochrane Database of Systematic Reviews (CDSR)] Retrieved 2016-07-20.</ref> There are six types of Cochrane Review:<ref>[http://tech.cochrane.org/revman Review Manager (RevMan) [Computer program<nowiki>]</nowiki>]. Version 5.2. Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2012.</ref><ref>[http://www.cochranelibrary.com/ The Cochrane Library]</ref><ref>{{cite journal | vauthors = Silva V, Grande AJ, Carvalho AP, Martimbianco AL, Riera R | title = Overview of systematic reviews - a new type of study. Part II | journal = Sao Paulo Medical Journal | volume = 133 | issue = 3 | pages = 206–17 | year = 2015 | pmid = 25388685 | doi = 10.1590/1516-3180.2013.8150015 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Silva V, Grande AJ, Martimbianco AL, Riera R, Carvalho AP | title = Overview of systematic reviews - a new type of study: part I: why and for whom? | journal = Sao Paulo Medical Journal | volume = 130 | issue = 6 | pages = 398–404 | year = 2012 | pmid = 23338737 | doi = 10.1590/S1516-31802012000600007 | doi-access = free }}</ref>
# ''Intervention reviews'' assess the benefits and harms of interventions used in healthcare and health policy.
# ''Diagnostic test accuracy reviews'' assess how well a diagnostic test performs in diagnosing and detecting a particular disease.
# ''Methodology reviews'' address issues relevant to how systematic reviews and clinical trials are conducted and reported.
# ''Qualitative reviews'' synthesize qualitative and quantitative evidence to address questions on aspects other than effectiveness.<ref name=Bearman2013 />
# ''Prognosis reviews'' address the probable course or future outcome(s) of people with a health problem.
# ''Overviews of systematic reviews'' (OoRs) are a new type of study in order to compile multiple evidence from systematic reviews into a single document that is accessible and useful to serve as a friendly front end for the Cochrane Collaboration with regard to healthcare decision-making.


The Cochrane Library is owned by Cochrane. It was originally published by Update Software and now published by the share-holder owned publisher [[John Wiley & Sons]], Ltd. as part of Wiley Online Library. Royalties from sales of the Cochrane Library are the major source of funds for Cochrane (over £6 million in 2017). There are 3.66 billion people around the world who have access to the Library through national licences (national licences cost £1.5 billion<ref name=":6">{{Cite web|last=|first=|date=2019-07-02|title=Cochrane’s Future Publishing and Open Access Arrangements|url=https://www.cochrane.org/news/cochranes-future-publishing-and-open-access-arrangements|archive-url=https://web.archive.org/web/20190702111124/https://www.cochrane.org/news/cochranes-future-publishing-and-open-access-arrangements|archive-date=2019-07-02|access-date=|website=Cochrane.org}}</ref>) or free provision for populations in low- and middle-income countries eligible under the WHO’s HINARI initiative.<ref name=":6" /> Authors must pay an additional fee for their review to be truly [[open access]].<ref>{{cite web|title=Open access options for the Cochrane Database of Systematic Reviews|url=https://www.cochranelibrary.com/about/open-access|access-date=2019-02-24|work=Cochrane}}</ref> Cochrane has an annual income of $10m USD.<ref>{{cite journal|vauthors=Newman M|date=January 2019|title=Has Cochrane lost its way?|journal=BMJ|volume=364|pages=k5302|doi=10.1136/bmj.k5302|pmid=30606713}}</ref>
The Cochrane Collaboration provides a handbook for systematic reviewers of interventions which "provides guidance to authors for the preparation of Cochrane Intervention reviews."<ref name=CochraneHandbook/> The ''Cochrane Handbook'' outlines eight general steps for preparing a systematic review:<ref name=CochraneHandbook/>
# Defining the review question(s) and developing criteria for including studies
# Searching for studies
# Selecting studies and collecting data
# Assessing risk of bias in included studies
# Analysing data and undertaking meta-analyses
# Addressing [[reporting bias]]es
# Presenting results and "summary of findings" tables
# Interpreting results and drawing conclusions


==== Public involvement and citizen science in systematic reviews ====
The Cochrane Handbook forms the basis of two sets of standards for the conduct and reporting of Cochrane Intervention Reviews (MECIR - Methodological Expectations of Cochrane Intervention Reviews)<ref>{{cite web |url= http://www.editorial-unit.cochrane.org/mecir |title= Methodological Expectations of Cochrane Intervention Reviews (MECIR) |publisher=Cochrane |access-date=6 October 2014}}</ref>
Cochrane has several tasks that the public or other 'stakeholders' can be involved in doing, associated with producing systematic reviews and other outputs. Tasks can be organised as 'entry level' or higher. Tasks include:


* Joining a collaborative volunteer effort to help categorise and summarise healthcare evidence<ref>{{cite web|title=Cochrane crowd|url=http://crowd.cochrane.org/index.html|access-date=2019-02-14|website=crowd.cochrane.org}}</ref>
The [[Cochrane Library]] is a collection of [[database]]s in medicine and other [[health care]] specialties provided by [[Cochrane (organisation)|Cochrane]] and other organizations. It is the collection of ''Cochrane Reviews'', a database of systematic reviews and [[Meta-analysis|meta-analyses]] which summarize and interpret the results of medical research. It was originally published by Update Software and now published by the share-holder owned publisher [[John Wiley & Sons]], Ltd. as part of Wiley Online Library.
* Data extraction and risk of bias assessment
* Translation of reviews into other languages


A recent systematic review of how people were involved in systematic reviews aimed to document the evidence-base relating to stakeholder involvement in systematic reviews and to use this evidence to describe how stakeholders have been involved in systematic reviews.<ref>{{cite journal|vauthors=Pollock A, Campbell P, Struthers C, Synnot A, Nunn J, Hill S, Goodare H, Morris J, Watts C, Morley R|date=November 2018|title=Stakeholder involvement in systematic reviews: a scoping review|journal=Systematic Reviews|volume=7|issue=1|pages=208|doi=10.1186/s13643-018-0852-0|pmc=6260873|pmid=30474560}}</ref> Thirty percent involved patients and/or carers. The ACTIVE framework provides a way to consistently describe how people are involved in systematic review, and may be used as a way to support the decision-making of systematic review authors in planning how to involve people in future reviews.<ref>{{Cite journal|last=Pollock|first=Alex|last2=Campbell|first2=Pauline|last3=Struthers|first3=Caroline|last4=Synnot|first4=Anneliese|last5=Nunn|first5=Jack|last6=Hill|first6=Sophie|last7=Goodare|first7=Heather|last8=Morris|first8=Jacqui|last9=Watts|first9=Chris|date=2019-04-18|title=Development of the ACTIVE framework to describe stakeholder involvement in systematic reviews|url=https://doi.org/10.1177/1355819619841647|journal=Journal of Health Services Research & Policy|language=en|pages=135581961984164|doi=10.1177/1355819619841647|issn=1355-8196}}</ref> Standardised Data on Initiatives (STARDIT) is another proposed way of reporting who has been involved in which tasks during research, including systematic reviews.<ref>[Pre-print] {{Cite web|title=Standardised Data on Initiatives - STARDIT: Alpha Version|url=https://osf.io/5q47h/|access-date=2020-08-20|website=osf.io|doi=10.31219/osf.io/5q47h}}</ref>
Authors must pay an additional fee for their review to be truly [[open access]].<ref>{{cite web|url=https://www.cochranelibrary.com/about/open-access|title=Open access options for the Cochrane Database of Systematic Reviews |work = Cochrane |access-date=2019-02-24}}</ref> Cochrane has an annual income of $10m USD.<ref name="Has Cochrane lost its way">{{cite journal | vauthors = Newman M | title = Has Cochrane lost its way? | journal = BMJ | volume = 364 | pages = k5302 | date = January 2019 | doi = 10.1136/bmj.k5302 | s2cid = 58623482 }}</ref>


While there has been some criticism of how Cochrane prioritises systematic reviews,<ref>{{Cite journal|last=Newman|first=Melanie|date=2019-01-03|title=Has Cochrane lost its way?|url=https://www.bmj.com/content/364/bmj.k5302|journal=BMJ|language=en|volume=364|pages=k5302|doi=10.1136/bmj.k5302|issn=0959-8138|pmid=30606713}}</ref> a recent project involved people in helping identify research priorities to inform future Cochrane Reviews.<ref>{{Cite journal|last=Synnot|first=Anneliese J.|last2=Tong|first2=Allison|last3=Bragge|first3=Peter|last4=Lowe|first4=Dianne|last5=Nunn|first5=Jack S.|last6=O’Sullivan|first6=Molly|last7=Horvat|first7=Lidia|last8=Kay|first8=Debra|last9=Ghersi|first9=Davina|date=2019-04-29|title=Selecting, refining and identifying priority Cochrane Reviews in health communication and participation in partnership with consumers and other stakeholders|url=https://doi.org/10.1186/s12961-019-0444-z|journal=Health Research Policy and Systems|volume=17|issue=1|pages=45|doi=10.1186/s12961-019-0444-z|issn=1478-4505|pmc=PMC6489310|pmid=31036016}}</ref><ref>{{cite journal|vauthors=Synnot A, Bragge P, Lowe D, Nunn JS, O'Sullivan M, Horvat L, Tong A, Kay D, Ghersi D, McDonald S, Poole N, Bourke N, Lannin N, Vadasz D, Oliver S, Carey K, Hill SJ|date=May 2018|title=Research priorities in health communication and participation: international survey of consumers and other stakeholders|journal=BMJ Open|volume=8|issue=5|pages=e019481|doi=10.1136/bmjopen-2017-019481|pmc=5942413|pmid=29739780}}</ref> In 2014, the Cochrane-Wikipedia partnership was formalised. This supports the inclusion of relevant evidence within all Wikipedia medical articles, as well as other processes to help ensure that medical information included in Wikipedia is of the highest quality and accuracy.<ref>{{cite web|title=The Cochrane-Wikipedia partnership in 2016|url=https://www.cochrane.org/news/cochrane-wikipedia-partnership-2016-0|access-date=2019-02-24|work=Cochrane}}</ref>
=== Social sciences ===
The quasi-standard for systematic review in the social sciences is based on the procedures proposed by the [[Campbell Collaboration]], which is one of a number of groups promoting [[evidence-based policy]] in the [[social science]]s. The Campbell Collaboration "helps people make well-informed decisions by preparing, maintaining and disseminating systematic reviews in education, crime and justice, social welfare and international development.<ref>{{cite web|url = http://www.campbellcollaboration.org/about_us/index.php|title = About Us|publisher = The Campbell Collaboration|access-date = March 26, 2014|url-status = dead|archive-url = https://web.archive.org/web/20140203232624/http://www.campbellcollaboration.org/about_us/index.php|archive-date = February 3, 2014}}</ref> It is a sister initiative of Cochrane. The Campbell Collaboration was created in 2000 and the inaugural meeting in Philadelphia, USA, attracted 85 participants from 13 countries.<ref>{{cite web |url= http://www.campbellcollaboration.org/history/explore/background | archive-url = https://web.archive.org/web/20160924070231/http://www.campbellcollaboration.org/history/explore/background | archive-date = 24 September 2016 | url-status = dead |title=History - Campbell |website=www.campbellcollaboration.org |access-date=2016-09-27}}</ref>


==== Learning resources ====
In 1959, social scientist and social work educator [[Barbara Wootton]] published one of the first contemporary systematic reviews of literature on anti-social behavior as part of her work, ''Social Science and Social Pathology''.<ref>{{cite book |last1=Gough |first1=David |last2=Oliver |first2=Sandy |last3=Thomas |first3=James |title=An Introduction to Systematic Reviews |date=2017 |publisher=Sage |location=London |page=XIII |edition=2nd}}</ref><ref>{{cite book |last1=Oakley |first1=Ann |title=A critical woman : Barbara Wootton, social science and public policy in the twentieth century |date=2011 |publisher=Bloomsbury Academic |location=London |isbn=9781849664707}}</ref>
Cochrane has produced many learning resources to help people understand what systematic reviews are, and how to do them. Most of the learning resources can be found at the 'Cochrane Training' webpage,<ref>{{Cite web|title=Welcome|url=https://training.cochrane.org/|access-date=2019-06-25|website=training.cochrane.org|language=en}}</ref> which also includes a link to the book ''Testing Treatments'', which has been translated into many languages.<ref>{{Cite web|date=2019-06-25|title=Testing Treatments|url=https://training.cochrane.org/online-learning/knowledge-translation/testing-treatments|archive-url=https://web.archive.org/web/20190625002135/https://training.cochrane.org/online-learning/knowledge-translation/testing-treatments|archive-date=2019-06-25|access-date=2019-06-25|website=Cochrane Training}}</ref> In addition, Cochrane has created a short video ''What are Systematic Reviews'' which explains in plain English how they work and what they are used for.<ref>{{Citation|last=Cochrane|title=What are systematic reviews?|date=2016-01-27|url=https://www.youtube.com/watch?v=egJlW4vkb1Y&feature=youtu.be|accessdate=2019-06-25}}</ref> The video has been translated into multiple languages,<ref>{{Cite journal|date=2016-10-23|title=Successful multi-language free online animated learning resource|url=http://doi.wiley.com/10.1002/14651858.CD201602|journal=Abstracts of the 24th Cochrane Colloquium; 23–27 October 2016, Seoul, South Korea|publisher=Wiley|doi=10.1002/14651858.CD201602}}</ref> and viewed over 192,282 times (as of August 2020). In addition, an animated storyboard version was produced and all the video resources were released in multiple versions under Creative Commons for others to use and adapt.<ref>{{Cite web|date=2019-06-25|title=Creating a successful online animation resource: the ‘what are systematic reviews?’ video|url=https://community.cochrane.org/news/creating-successful-online-animation-resource-what-are-systematic-reviews-video|archive-url=https://web.archive.org/web/20190625001803/https://community.cochrane.org/news/creating-successful-online-animation-resource-what-are-systematic-reviews-video|archive-date=2019-06-25|access-date=2019-06-25|website=Cochrane Community}}</ref><ref name="Cochrane storyboard">{{cite web|url=https://cccrg.cochrane.org/animated-storyboard-what-are-systematic-reviews|title=Animated Storyboard: What Are Systematic Reviews?|website=cccrg.cochrane.org|publisher=Cochrane Consumers and Communication|access-date=1 June 2016}}</ref><ref>{{Cite web|last=Nunn|first=Jack|last2=Synnot|first2=Anneliese|last3=Mcdonald|first3=Steve|last4=Allen|first4=Kelly|last5=Hill|first5=Sophie|date=2015|title=Building partnerships with the public by learning about Cochrane evidence|url=https://doi.org/10.13140/RG.2.1.2182.1922|archive-url=https://archive.org/details/CochranePosterPrintV1717915|archive-date=2015-11-09|publisher=23rd Cochrane Colloquium 2015|language=en}}</ref><ref>{{cite web|last1=Nunn|first1=Jack|last2=Hill|first2=Sophie|title=What are systematic reviews?|url=https://archive.org/details/WhatAreSystematicReviews|accessdate=2020-08-20|language=en}}</ref> The Critical Appraisal Skills Programme (CASP) provides free learning resources to support people to appraise research critically, including a checklist which contains 10 questions to 'help you make sense of a systematic review'.<ref>{{Cite web|title=CASP Checklists|url=https://casp-uk.net/casp-tools-checklists/|access-date=2020-08-21|website=Critical Appraisal Skills Programme|language=en}}</ref><ref>{{Cite web|date=2020-08-21|title=CASP Systematic Review Checklist|url=https://casp-uk.net/wp-content/uploads/2018/03/CASP-Systematic-Review-Checklist-2018_fillable-form.pdf|archive-url=https://web.archive.org/web/20200821063301/https://casp-uk.net/wp-content/uploads/2018/03/CASP-Systematic-Review-Checklist-2018_fillable-form.pdf|archive-date=2020-08-21|access-date=2020-08-21|website=casp-uk.net}}</ref>

=== Social, behavioural and educational ===
Several organisations use systematic reviews in social, behavioural, and educational areas of evidence-based policy, including the National Institute for Health and Care Excellence (NICE, UK), [[wikipedia:Social_Care_Institute_for_Excellence|Social Care Institute for Excellence]] (SCIE, UK), the Agency for Healthcare Research and Quality (AHRQ, USA), the World Health Organisation, the International Initiative for Impact Evaluation (3ie), the [[wikipedia:The_Joanna_Briggs_Institute|Joanna Briggs Institute]] and the [[Campbell Collaboration]]. The quasi-standard for systematic review in the social sciences is based on the procedures proposed by the Campbell Collaboration, which is one of several groups promoting [[evidence-based policy]] in the [[Social science|social sciences]]. The Campbell Collaboration: 'helps people make well-informed decisions by preparing, maintaining and disseminating systematic reviews in education, crime and justice, social welfare and international development.'<ref>{{cite web|title=About Us|url=http://www.campbellcollaboration.org/about_us/index.php|archive-url=https://web.archive.org/web/20140203232624/http://www.campbellcollaboration.org/about_us/index.php|archive-date=February 3, 2014|access-date=March 26, 2014|publisher=The Campbell Collaboration}}</ref> The Campbell Collaboration is a sibling initiative of Cochrane, and was created in 2000 at the inaugural meeting in Philadelphia, USA, attracting 85 participants from 13 countries.<ref>{{cite web|title=History - Campbell|url=http://www.campbellcollaboration.org/history/explore/background|archive-url=https://web.archive.org/web/20160924070231/http://www.campbellcollaboration.org/history/explore/background|archive-date=24 September 2016|access-date=2016-09-27|website=www.campbellcollaboration.org}}</ref>


=== Business and economics ===
=== Business and economics ===
Due to the different nature of research fields outside of the natural sciences, the aforementioned methodological steps cannot easily be applied in all areas of business research. Some attempts to transfer the procedures from medicine to business research have been made,<ref>{{cite journal|vauthors=Tranfield D, Denyer D, Smart P|year=2003|title=Towards a methodology for developing evidence-informed management knowledge by means of systematic review|journal=British Journal of Management|volume=14|issue=3|pages=207–222|citeseerx=10.1.1.622.895|doi=10.1111/1467-8551.00375}}</ref> including a step-by-step approach,<ref>{{cite journal|vauthors=Durach CF, Kembro J, Wieland A|year=2017|title=A New Paradigm for Systematic Literature Reviews in Supply Chain Management|journal=Journal of Supply Chain Management|volume=53|issue=4|pages=67–85|doi=10.1111/jscm.12145}}</ref> and developing a standard procedure for conducting systematic literature reviews in business and economics. The Campbell & Cochrane Economics Methods Group (C-CEMG) works to improve the inclusion of economic evidence into Cochrane and Campbell systematic reviews of interventions, to enhance the usefulness of review findings as a component for decision-making.<ref>{{Cite web|title=About us|url=https://methods.cochrane.org/economics/about-us|access-date=2020-07-01|website=methods.cochrane.org|language=en}}</ref> Such economic evidence is crucial for health technology assessment processes.


=== International development research ===
Due to the different nature of research fields outside of the natural sciences, the aforementioned methodological steps cannot easily be applied in business research. Early attempts to transfer the procedures from medicine to business research have been made by Tranfield et al. (2003).<ref>{{cite journal | vauthors = Tranfield D, Denyer D, Smart P | year = 2003 | title = Towards a methodology for developing evidence-informed management knowledge by means of systematic review | journal = British Journal of Management | volume = 14 | issue = 3| pages = 207–222 | doi=10.1111/1467-8551.00375| citeseerx = 10.1.1.622.895 }}</ref> A step-by-step approach has been developed by Durach et al.: Based on the experiences they have made in their own discipline, these authors have adapted the methodological steps and developed a standard procedure for conducting systematic literature reviews in business and economics.<ref>{{cite journal | vauthors = Durach CF, Kembro J, Wieland A | year = 2017 | title = A New Paradigm for Systematic Literature Reviews in Supply Chain Management | journal = Journal of Supply Chain Management | volume = 53 | issue = 4| pages = 67–85 | doi=10.1111/jscm.12145 }}</ref>
Systematic reviews are increasingly prevalent in other fields, such as international development research.<ref name="Hagen-Zanker20122">{{cite web|last1=Hagen-Zanker|first1=Jessica|last2=Duvendack|first2=Maren|last3=Mallett|first3=Richard|last4=Slater|first4=Rachel|last5=Carpenter|first5=Samuel|last6=Tromme|first6=Mathieu|date=January 2012|title=Making systematic reviews work for international development research|url=http://www.odi.org.uk/resources/details.asp?id=6260&title=systematic-review-slrc-international-development-research-methods|publisher=Overseas Development Institute|name-list-format=vanc}}</ref> Subsequently, several donors (including the UK Department for International Development (DFID) and [[AusAid]]) are focusing more attention and resources on testing the appropriateness of systematic reviews in assessing the impacts of development and humanitarian interventions.<ref name="Hagen-Zanker20122" />


=== Environment ===
==Strengths and weaknesses==
The Collaboration for Environmental Evidence (CEE) works to achieve a sustainable global environment and the conservation of biodiversity. The CEE has a journal titled ''Environmental Evidence'' which publishes systematic reviews, review protocols and systematic maps on impacts of human activity and the effectiveness of management interventions.<ref>{{Cite web|title=Environmental Evidence: Reliable evidence, informed decisions, better environment|url=https://www.environmentalevidence.org/|access-date=2020-07-01|website=www.environmentalevidence.org}}</ref>
While systematic reviews are regarded as the strongest form of medical evidence, a review of 300 studies found that not all systematic reviews were equally reliable, and that their reporting can be improved by a universally agreed upon set of standards and guidelines.<ref>{{cite journal | vauthors = Moher D, Tetzlaff J, Tricco AC, Sampson M, Altman DG | title = Epidemiology and reporting characteristics of systematic reviews | journal = PLOS Medicine | volume = 4 | issue = 3 | pages = e78 | date = March 2007 | pmid = 17388659 | pmc = 1831728 | doi = 10.1371/journal.pmed.0040078 }}</ref> A further study by the same group found that of 100 systematic reviews monitored, 7% needed updating at the time of publication, another 4% within a year, and another 11% within 2 years; this figure was higher in rapidly changing fields of medicine, especially cardiovascular medicine.<ref name="pmid17638714">{{cite journal | vauthors = Shojania KG, Sampson M, Ansari MT, Ji J, Doucette S, Moher D | title = How quickly do systematic reviews go out of date? A survival analysis | journal = Annals of Internal Medicine | volume = 147 | issue = 4 | pages = 224–33 | date = August 2007 | pmid = 17638714 | doi = 10.7326/0003-4819-147-4-200708210-00179 | doi-access = free }}</ref> A 2003 study suggested that extending searches beyond major databases, perhaps into [[grey literature]], would increase the effectiveness of reviews.<ref>{{cite journal | vauthors = Savoie I, Helmer D, Green CJ, Kazanjian A | title = Beyond Medline: reducing bias through extended systematic review search | journal = International Journal of Technology Assessment in Health Care | volume = 19 | issue = 1 | pages = 168–78 | year = 2003 | pmid = 12701949 | doi = 10.1017/S0266462303000163 }}</ref> Additionally, errors in abstracting study design and baseline characteristics can be reduced through adjudication or independent abstraction, yet that does not adequately handle inconsistencies in data related to outcomes or results.<ref>{{Cite journal|last1=E|first1=Jian‐Yu|last2=Saldanha|first2=Ian J.|last3=Canner|first3=Joseph|last4=Schmid|first4=Christopher H.|last5=Le|first5=Jimmy T.|last6=Li|first6=Tianjing|date=2020|title=Adjudication rather than experience of data abstraction matters more in reducing errors in abstracting data in systematic reviews|journal=Research Synthesis Methods|language=en|volume=11|issue=3|pages=354–362|doi=10.1002/jrsm.1396|pmid=31955502|issn=1759-2879}}</ref>


== Review tools ==
Roberts and colleagues highlighted the problems with systematic reviews, particularly those conducted by the [[Cochrane (organisation)|Cochrane]], noting that published reviews are often biased, out of date and excessively long.<ref>{{cite journal | vauthors = Roberts I, Ker K, Edwards P, Beecher D, Manno D, Sydenham E | title = The knowledge system underpinning healthcare is not fit for purpose and must change | journal = BMJ | volume = 350 | pages = h2463 | date = June 2015 | pmid = 26041754 | doi = 10.1136/bmj.h2463 | s2cid = 33523612 | url = http://researchonline.lshtm.ac.uk/2210686/1/bmj.h2463.full.pdf }}</ref> They criticized Cochrane reviews as not being sufficiently critical in the selection of trials and including too many of low quality. They proposed several solutions, including limiting studies in meta-analyses and reviews to registered clinical trials, requiring that original data be made available for statistical checking, paying greater attention to sample size estimates, and eliminating dependence on only published data.
A 2019 publication identified 15 systematic review tools and ranked them according to the number of 'critical features' as required to perform a systematic review, including:<ref>{{Cite journal|last=van der Mierden|first=Stevie|date=2019|title=Software tools for literature screening in systematic reviews in biomedical research|url=https://www.altex.org/index.php/altex/article/view/1257|journal=ALTEX|volume=|pages=|doi=10.14573/altex.1902131|pmid=31113000|via=}}</ref>


* DistillerSR: a paid web application
Some of these difficulties were noted early on as described by Altman: "much poor research arises because researchers feel compelled for career reasons to carry out research that they are ill equipped to perform, and nobody stops them."<ref>{{cite journal | vauthors = Altman DG | title = The scandal of poor medical research | journal = BMJ | volume = 308 | issue = 6924 | pages = 283–4 | date = January 1994 | pmid = 8124111 | pmc = 2539276 | doi = 10.1136/bmj.308.6924.283 }}</ref> Methodological limitations of meta-analysis have also been noted.<ref>{{cite journal | vauthors = Shapiro S | title = Meta-analysis/Shmeta-analysis | journal = American Journal of Epidemiology | volume = 140 | issue = 9 | pages = 771–8 | date = November 1994 | pmid = 7977286 | doi = 10.1093/oxfordjournals.aje.a117324 }}</ref> Until now, there was no standardized methods to be used by researchers and the optimal methodological steps should be standardized.<ref name=Giang/> Another concern is that the methods used to conduct a systematic review are sometimes changed once researchers see the available trials they are going to include.<ref>{{cite journal | vauthors = Page MJ, McKenzie JE, Kirkham J, Dwan K, Kramer S, Green S, Forbes A | title = Bias due to selective inclusion and reporting of outcomes and analyses in systematic reviews of randomised trials of healthcare interventions | journal = The Cochrane Database of Systematic Reviews | issue = 10 | pages = MR000035 | date = October 2014 | pmid = 25271098 | doi = 10.1002/14651858.MR000035.pub2 }}</ref> Bloggers have described retractions of systematic reviews and published reports of studies included in published systematic reviews.<ref>{{cite web | first = Ian | last = Roberts | name-list-style = vanc |title= Retraction Of Scientific Papers For Fraud Or Bias Is Just The Tip Of The Iceberg |url=http://www.iflscience.com/editors-blog/retraction-scientific-papers-fraud-or-bias-just-tip-iceberg|website=IFL Science!|access-date=29 June 2015}}</ref><ref>{{cite web | first = Cat | last = Ferguson | name-list-style = vanc |title=Retraction and republication for Lancet Resp Med tracheostomy paper |url= http://retractionwatch.com/2015/04/02/retraction-and-republication-for-lancet-resp-med-tracheostomy-paper/| work = Retraction Watch |access-date=29 June 2015 |date=2015-04-02 }}</ref><ref>{{cite web | first = Cat | last = Ferguson | name-list-style = vanc |title=BioMed Central retracting 43 papers for fake peer review |url= http://retractionwatch.com/2015/03/26/biomed-central-retracting-43-papers-for-fake-peer-review/ |work=Retraction Watch|date=2015-03-26 }}</ref>
* Swift Active Screener: a paid web application
* Covidence: a paid web application and Cochrane technology platform.
* Rayyan: a free web application
* Sysrev: a free web application


== Limitations ==
Systematic reviews are increasingly prevalent in other fields, such as international development research.<ref name=Hagen-Zanker2012>{{cite web |first1=Jessica |last1=Hagen-Zanker |first2=Maren |last2=Duvendack |first3=Richard |last3=Mallett |first4=Rachel |last4=Slater |first5=Samuel |last5=Carpenter |first6=Mathieu |last6=Tromme | name-list-style = vanc |date=January 2012 |title=Making systematic reviews work for international development research |url=http://www.odi.org.uk/resources/details.asp?id=6260&title=systematic-review-slrc-international-development-research-methods |publisher=[[Overseas Development Institute]]}}</ref> Subsequently, a number of donors – most notably the UK Department for International Development (DFID) and [[AusAid]] – are focusing more attention and resources on testing the appropriateness of systematic reviews in assessing the impacts of development and humanitarian interventions.<ref name=Hagen-Zanker2012/>


=== Limited reporting of clinical trials ===
=== Out-dated or risk of bias ===
While systematic reviews are regarded as the strongest form of evidence, a 2003 review of 300 studies found that not all systematic reviews were equally reliable, and that their reporting can be improved by a universally agreed upon set of standards and guidelines.<ref>{{cite journal|vauthors=Moher D, Tetzlaff J, Tricco AC, Sampson M, Altman DG|date=March 2007|title=Epidemiology and reporting characteristics of systematic reviews|journal=PLoS Medicine|volume=4|issue=3|pages=e78|doi=10.1371/journal.pmed.0040078|pmc=1831728|pmid=17388659}}</ref> A further study by the same group found that of 100 systematic reviews monitored, 7% needed updating at the time of publication, another 4% within a year, and another 11% within 2 years; this figure was higher in rapidly changing fields of medicine, especially cardiovascular medicine.<ref name="pmid176387142">{{cite journal|vauthors=Shojania KG, Sampson M, Ansari MT, Ji J, Doucette S, Moher D|date=August 2007|title=How quickly do systematic reviews go out of date? A survival analysis|journal=Annals of Internal Medicine|volume=147|issue=4|pages=224–33|doi=10.7326/0003-4819-147-4-200708210-00179|pmid=17638714}}</ref> A 2003 study suggested that extending searches beyond major databases, perhaps into [[grey literature]], would increase the effectiveness of reviews.<ref>{{cite journal|vauthors=Savoie I, Helmer D, Green CJ, Kazanjian A|year=2003|title=Beyond Medline: reducing bias through extended systematic review search|journal=International Journal of Technology Assessment in Health Care|volume=19|issue=1|pages=168–78|doi=10.1017/S0266462303000163|pmid=12701949}}</ref>
The 'All Trials' campaign highlights that around half of clinical trials have never reported results and works to improve reporting.<ref>{{Cite web|url=http://www.alltrials.net/news/half-of-all-trials-unreported/|title=Half of all clinical trials have never reported results|date=20 August 2015|website=AllTrials|access-date=2019-03-08}}</ref> This lack of reporting has extremely serious implications for research, including systematic reviews, as it is only possible to synthesize data of published trials. In addition, positive trials were twice as likely to be published as those with negative results.<ref>{{Cite journal|last1=Song|first1=F.|last2=Parekh|first2=S.|last3=Hooper|first3=L.|last4=Loke|first4=Y. K.|last5=Ryder|first5=J.|last6=Sutton|first6=A. J.|last7=Hing|first7=C.|last8=Kwok|first8=C. S.|last9=Pang|first9=C.|date=February 2010|title=Dissemination and publication of research findings: an updated review of related biases|journal=Health Technology Assessment |volume=14|issue=8|pages=iii, ix–xi, 1–193|doi=10.3310/hta14080|pmid=20181324|name-list-style=vanc|doi-access=free}}</ref> At present, it is legal for for-profit companies to conduct clinical trials and not publish the results.<ref name="Iacobucci i5955">{{Cite journal|last=Iacobucci|first=Gareth|date=2016-11-04|title=Nearly half of all trials run by major sponsors in past decade are unpublished|journal=BMJ|volume=355|pages=i5955|doi=10.1136/bmj.i5955|pmid=27815253|s2cid=43604202|name-list-style=vanc}}</ref> For example, in the past 10 years 8.7 million patients have taken part in trials that have not published results.<ref name="Iacobucci i5955"/> These factors mean that it is likely there is a significant publication bias, with only 'positive' or perceived favorable results being published. A recent systematic review of industry sponsorship and research outcomes concluded that 'sponsorship of drug and device studies by the manufacturing company leads to more favorable efficacy results and conclusions than sponsorship by other sources' and that the existence of an industry bias that cannot be explained by standard 'Risk of bias' assessments.<ref>{{Cite journal|last1=Lundh|first1=Andreas|last2=Lexchin|first2=Joel|last3=Mintzes|first3=Barbara|last4=Schroll|first4=Jeppe B|last5=Bero|first5=Lisa|date=2017-02-16|title=Industry sponsorship and research outcome|journal=Cochrane Database of Systematic Reviews|volume=2|pages=MR000033|doi=10.1002/14651858.MR000033.pub3|pmid=28207928|name-list-style=vanc|url=https://portal.findresearcher.sdu.dk/da/publications/9a5fd1d6-2b2e-4f81-8014-e03cf1977f89}}</ref>


Some authors have highlighted problems with systematic reviews, particularly those conducted by [[Cochrane (organisation)|Cochrane]], noting that published reviews are often biased, out of date and excessively long.<ref>{{cite journal|vauthors=Roberts I, Ker K, Edwards P, Beecher D, Manno D, Sydenham E|date=June 2015|title=The knowledge system underpinning healthcare is not fit for purpose and must change|journal=BMJ|volume=350|pages=h2463|doi=10.1136/bmj.h2463|pmid=26041754}}</ref> Cochrane reviews have been criticized as not being sufficiently critical in the selection of trials and including too many of low quality. They proposed several solutions, including limiting studies in meta-analyses and reviews to [[Clinical trials registry|registered clinical trials]], requiring that original data be made available for statistical checking, paying greater attention to sample size estimates, and eliminating dependence on only published data.
Systematic reviews of such a bias may amplify the effect, although the flaw is in the reporting of research, not in the systematic review process.


Some of these difficulties were noted as early as 1994:
== Review tools ==
{{quote|much poor research arises because researchers feel compelled for career reasons to carry out research that they are ill equipped to perform, and nobody stops them.}}
A 2019 publication identified 15 systematic review tools and ranked them according to the number of 'critical features' as required to perform a systematic review, including:<ref>{{Cite journal|last=van der Mierden|first=Stevie|date=2019|title=Software tools for literature screening in systematic reviews in biomedical research|journal=ALTEX|volume=36|issue=3|pages=508–517|doi=10.14573/altex.1902131|pmid=31113000|name-list-style=vanc|doi-access=free}}</ref>
''DG Altman, 1994''<ref>{{cite journal|vauthors=Altman DG|date=January 1994|title=The scandal of poor medical research|journal=BMJ|volume=308|issue=6924|pages=283–4|doi=10.1136/bmj.308.6924.283|pmc=2539276|pmid=8124111}}</ref>

Methodological limitations of meta-analysis have also been noted.<ref>{{cite journal|vauthors=Shapiro S|date=November 1994|title=Meta-analysis/Shmeta-analysis|journal=American Journal of Epidemiology|volume=140|issue=9|pages=771–8|doi=10.1093/oxfordjournals.aje.a117324|pmid=7977286}}</ref> Another concern is that the methods used to conduct a systematic review are sometimes changed once researchers see the available trials they are going to include.<ref>{{cite journal|vauthors=Page MJ, McKenzie JE, Kirkham J, Dwan K, Kramer S, Green S, Forbes A|date=October 2014|title=Bias due to selective inclusion and reporting of outcomes and analyses in systematic reviews of randomised trials of healthcare interventions|journal=The Cochrane Database of Systematic Reviews|issue=10|pages=MR000035|doi=10.1002/14651858.MR000035.pub2|pmid=25271098}}</ref> Some website have described retractions of systematic reviews and published reports of studies included in published systematic reviews.<ref>{{cite web|last=Roberts|first=Ian|title=Retraction Of Scientific Papers For Fraud Or Bias Is Just The Tip Of The Iceberg|url=http://www.iflscience.com/editors-blog/retraction-scientific-papers-fraud-or-bias-just-tip-iceberg|access-date=29 June 2015|website=IFL Science!|name-list-format=vanc}}</ref><ref>{{cite web|last=Ferguson|first=Cat|date=2015-04-02|title=Retraction and republication for Lancet Resp Med tracheostomy paper|url=http://retractionwatch.com/2015/04/02/retraction-and-republication-for-lancet-resp-med-tracheostomy-paper/|access-date=29 June 2015|work=Retraction Watch|name-list-format=vanc}}</ref><ref>{{cite web|last=Ferguson|first=Cat|date=2015-03-26|title=BioMed Central retracting 43 papers for fake peer review|url=http://retractionwatch.com/2015/03/26/biomed-central-retracting-43-papers-for-fake-peer-review/|work=Retraction Watch|name-list-format=vanc}}</ref> Eligibility criteria must be justifiable and not arbitrary (for example, the date range searched) as this may affect the perceived quality of the review.<ref>{{Cite web|last=Library|first=Flinders University|title=Search Smart: Systematic Reviews: Methodology overview|url=https://flinders.libguides.com/c.php?g=220637&p=1460288|access-date=2020-09-16|website=flinders.libguides.com|language=en}}</ref><ref>{{Cite book|last=Pursell|first=Edward|url=https://www.worldcat.org/oclc/1182880684|title=How to Perform a Systematic Literature Review: a guide for healthcare researchers, practitioners and students.|last2=McCrae|first2=Niall|date=2020|publisher=Springer Nature|year=|isbn=978-3-030-49672-2|location=|pages=|oclc=1182880684}}</ref>

=== Limited reporting of clinical trials and data from human studies ===
The '[[AllTrials]]' campaign highlights that around half of clinical trials have never reported results and works to improve reporting.<ref>{{Cite web|last=|first=|date=2015-08-20|title=Half of all clinical trials have never reported results|url=http://www.alltrials.net/news/half-of-all-trials-unreported/|archive-url=|archive-date=|access-date=2019-03-08|website=AllTrials}}</ref> This lack of reporting has extremely serious implications for research, including systematic reviews, as it is only possible to synthesize data of published studies. In addition, 'positive' trials were twice as likely to be published as those with 'negative' results.<ref>{{Cite journal|last=Song|first=F.|last2=Parekh|first2=S.|last3=Hooper|first3=L.|last4=Loke|first4=Y. K.|last5=Ryder|first5=J.|last6=Sutton|first6=A. J.|last7=Hing|first7=C.|last8=Kwok|first8=C. S.|last9=Pang|first9=C.|date=February 2010|title=Dissemination and publication of research findings: an updated review of related biases|journal=Health Technology Assessment (Winchester, England)|volume=14|issue=8|pages=iii, ix–xi, 1–193|doi=10.3310/hta14080|issn=2046-4924|pmid=20181324}}</ref> At present, it is legal for for-profit companies to conduct clinical trials and not publish the results.<ref name="Iacobucci i59552">{{Cite journal|last=Iacobucci|first=Gareth|date=2016-11-04|title=Nearly half of all trials run by major sponsors in past decade are unpublished|url=https://www.bmj.com/content/355/bmj.i5955|journal=BMJ|volume=355|pages=i5955|doi=10.1136/bmj.i5955|issn=1756-1833|pmid=27815253}}</ref> For example, in the past 10 years 8.7 million patients have taken part in trials that have not published results.<ref name="Iacobucci i59552" /> These factors mean that it is likely there is a significant publication bias, with only 'positive' or perceived favourable results being published. A recent systematic review of industry sponsorship and research outcomes concluded that 'sponsorship of drug and device studies by the manufacturing company leads to more favorable efficacy results and conclusions than sponsorship by other sources' and that the existence of an industry bias that cannot be explained by standard 'Risk of bias' assessments.<ref>{{Cite journal|last=Lundh|first=Andreas|last2=Lexchin|first2=Joel|last3=Mintzes|first3=Barbara|last4=Schroll|first4=Jeppe B|last5=Bero|first5=Lisa|date=2017-02-16|editor-last=Cochrane Methodology Review Group|title=Industry sponsorship and research outcome|journal=Cochrane Database of Systematic Reviews|volume=2|pages=MR000033|doi=10.1002/14651858.MR000033.pub3|pmid=28207928}}</ref> Systematic reviews of such a bias may amplify the effect, although it is important to note that the flaw is in the reporting of research generally, not in the systematic review method.

=== Poor compliance with review reporting guidelines ===
The rapid growth of systematic reviews in recent years has been accompanied by the attendant issue of poor compliance with guidelines, particularly in areas such as declaration of registered study protocols, funding source declaration, risk of bias data, and description of clear study objectives.<ref>{{Cite journal|last=Pidgeon|first=Thomas Edward|last2=Wellstead|first2=Georgina|last3=Sagoo|first3=Harkiran|last4=Jafree|first4=Daniyal J.|last5=Fowler|first5=Alexander J.|last6=Agha|first6=Riaz A.|date=2016-10|title=An assessment of the compliance of systematic review articles published in craniofacial surgery with the PRISMA statement guidelines: A systematic review|url=https://linkinghub.elsevier.com/retrieve/pii/S1010518216301524|journal=Journal of Cranio-Maxillofacial Surgery|language=en|volume=44|issue=10|pages=1522–1530|doi=10.1016/j.jcms.2016.07.018}}</ref><ref>{{Cite journal|last=Lee|first=Seon-Young|last2=Sagoo|first2=Harkiran|last3=Whitehurst|first3=Katharine|last4=Wellstead|first4=Georgina|last5=Fowler|first5=Alexander J.|last6=Agha|first6=Riaz A.|last7=Orgill|first7=Dennis|date=2016-03-01|title=Compliance of Systematic Reviews in Plastic Surgery With the PRISMA Statement|url=http://archfaci.jamanetwork.com/article.aspx?doi=10.1001/jamafacial.2015.1726|journal=JAMA Facial Plastic Surgery|language=en|volume=18|issue=2|pages=101|doi=10.1001/jamafacial.2015.1726|issn=2168-6076}}</ref><ref>{{Cite journal|last=Bulters|first=D.|last2=Zolnourian|first2=A.|last3=Akhigbe|first3=T.|date=2017-05-01|title=Compliance of systematic reviews articles in brain arteriovenous malformation with PRISMA statement guidelines: Review of literature|url=https://www.jocn-journal.com/article/S0967-5868(16)31237-1/abstract|journal=Journal of Clinical Neuroscience|language=en|volume=39|pages=45–48|doi=10.1016/j.jocn.2017.02.016|issn=0967-5868|pmid=28246008}}</ref><ref>{{Cite journal|last=Lee|first=Seon-Young|last2=Sagoo|first2=Harkiran|last3=Farwana|first3=Reem|last4=Whitehurst|first4=Katharine|last5=Fowler|first5=Alex|last6=Agha|first6=Riaz|date=2017-12|title=Compliance of systematic reviews in ophthalmology with the PRISMA statement|url=https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/s12874-017-0450-1|journal=BMC Medical Research Methodology|language=en|volume=17|issue=1|doi=10.1186/s12874-017-0450-1|issn=1471-2288|pmc=PMC5745614|pmid=29281981}}</ref> A host of studies have identified weaknesses in the rigour and reproducibility of search strategies in systematic reviews.<ref>{{Cite journal|last=Koffel|first=Jonathan B.|last2=Rethlefsen|first2=Melissa L.|date=2016-09-26|editor-last=Thombs|editor-first=Brett D|title=Reproducibility of Search Strategies Is Poor in Systematic Reviews Published in High-Impact Pediatrics, Cardiology and Surgery Journals: A Cross-Sectional Study|url=http://dx.plos.org/10.1371/journal.pone.0163309|journal=PLOS ONE|language=en|volume=11|issue=9|pages=e0163309|doi=10.1371/journal.pone.0163309|issn=1932-6203|pmc=PMC5036875|pmid=27669416}}</ref><ref>{{Cite journal|last=Yoshii|first=Adriana|last2=Plaut|first2=Daphne A.|last3=McGraw|first3=Kathleen A.|last4=Anderson|first4=Margaret J.|last5=Wellik|first5=Kay E.|date=2009-1|title=Analysis of the reporting of search strategies in Cochrane systematic reviews|url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605027/|journal=Journal of the Medical Library Association : JMLA|volume=97|issue=1|pages=21–29|doi=10.3163/1536-5050.97.1.004|issn=1536-5050|pmc=PMCPMC2605027|pmid=19158999}}</ref><ref>{{Cite journal|last=Toews|first=Lorraine C.|date=2017-07-07|title=Compliance of systematic reviews in veterinary journals with Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) literature search reporting guidelines|url=http://jmla.mlanet.org/ojs/jmla/article/view/246|journal=Journal of the Medical Library Association|language=en|volume=105|issue=3|pages=233–239|doi=10.5195/jmla.2017.246|issn=1558-9439|pmc=PMC5490700|pmid=28670210}}</ref><ref>{{Cite journal|last=Mullins|first=Mary M.|last2=DeLuca|first2=Julia B.|last3=Crepaz|first3=Nicole|last4=Lyles|first4=Cynthia M.|date=2014|title=Reporting quality of search methods in systematic reviews of HIV behavioral interventions (2000–2010): are the searches clearly explained, systematic and reproducible?|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/jrsm.1098|journal=Research Synthesis Methods|language=en|volume=5|issue=2|pages=116–130|doi=10.1002/jrsm.1098|issn=1759-2887|pmc=PMC5861495|pmid=26052651}}</ref><ref>{{Cite journal|last=Briscoe|first=Simon|date=2018|title=A review of the reporting of web searching to identify studies for Cochrane systematic reviews|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/jrsm.1275|journal=Research Synthesis Methods|language=en|volume=9|issue=1|pages=89–99|doi=10.1002/jrsm.1275|issn=1759-2887}}</ref><ref>{{Cite journal|last=McIntosh|first=Heather M.|last2=Loke|first2=Yoon|last3=Golder|first3=Su|date=2008-05-01|title=Poor reporting and inadequate searches were apparent in systematic reviews of adverse effects|url=https://www.jclinepi.com/article/S0895-4356(07)00217-X/abstract|journal=Journal of Clinical Epidemiology|language=en|volume=61|issue=5|pages=440–448|doi=10.1016/j.jclinepi.2007.06.005|issn=0895-4356|pmid=18394536}}</ref> To remedy this issue, a new PRISMA guideline extension called PRISMA-S is being developed to improve the quality, reporting, and reproducibility of systematic review search strategies.<ref>{{Cite journal|last=Waffenschmidt|first=Siw|last2=Koffel|first2=Jonathan|last3=Kirtley|first3=Shona|last4=Ayala|first4=Ana Patricia|last5=Rethlefsen|first5=Melissa|date=2019-03-04|title=PRISMA-S PRISMA Search Reporting Extension|url=https://osf.io/ygn9w/|language=en|doi=10.17605/OSF.IO/YGN9W}}</ref><ref>[Pre-print] {{Cite web|title=Preferred Reporting Items for Systematic reviews and Meta-Analysis Search Extension (PRISMA-S) 2019: Explanation and Elaboration|url=https://osf.io/sfc38/|access-date=2019-03-20|website=osf.io|doi=10.31219/osf.io/sfc38}}</ref> Furthermore, tools and checklists for peer-reviewing search strategies have been created, such as the Peer Review of Electronic Search Strategies (PRESS) guidelines.<ref>{{Cite journal|last=Lefebvre|first=Carol|last2=Foerster|first2=Vicki|last3=Cogo|first3=Elise|last4=Salzwedel|first4=Douglas M.|last5=Sampson|first5=Margaret|last6=McGowan|first6=Jessie|date=2016-07-01|title=PRESS Peer Review of Electronic Search Strategies: 2015 Guideline Statement|url=https://www.jclinepi.com/article/S0895-4356(16)00058-5/abstract|journal=Journal of Clinical Epidemiology|language=en|volume=75|pages=40–46|doi=10.1016/j.jclinepi.2016.01.021|issn=0895-4356|pmid=27005575}}</ref>

A key challenge for using systematic reviews in clinical practice and healthcare policy is assessing the quality of a given review. Consequently, a range of appraisal tools to evaluate systematic reviews have been designed. The two most popular measurement instruments and scoring tools for systematic review quality assessment are AMSTAR 2 (a measurement tool to assess the methodological quality of systematic reviews)<ref>{{Cite journal|last=Shea|first=Beverley J.|last2=Reeves|first2=Barnaby C.|last3=Wells|first3=George|last4=Thuku|first4=Micere|last5=Hamel|first5=Candyce|last6=Moran|first6=Julian|last7=Moher|first7=David|last8=Tugwell|first8=Peter|last9=Welch|first9=Vivian|date=2017-09-21|title=AMSTAR 2: a critical appraisal tool for systematic reviews that include randomised or non-randomised studies of healthcare interventions, or both|url=https://www.bmj.com/content/358/bmj.j4008|journal=BMJ|language=en|volume=358|doi=10.1136/bmj.j4008|issn=0959-8138|pmid=28935701}}</ref><ref>{{Cite journal|last=Shea|first=Beverley J.|last2=Grimshaw|first2=Jeremy M.|last3=Wells|first3=George A.|last4=Boers|first4=Maarten|last5=Andersson|first5=Neil|last6=Hamel|first6=Candyce|last7=Porter|first7=Ashley C.|last8=Tugwell|first8=Peter|last9=Moher|first9=David|date=2007-02-15|title=Development of AMSTAR: a measurement tool to assess the methodological quality of systematic reviews|url=https://doi.org/10.1186/1471-2288-7-10|journal=BMC Medical Research Methodology|volume=7|issue=1|pages=10|doi=10.1186/1471-2288-7-10|issn=1471-2288|pmc=PMC1810543|pmid=17302989}}</ref><ref>{{Cite journal|last=Boers|first=Maarten|last2=Henry|first2=David A.|last3=Grimshaw|first3=Jeremy|last4=Kristjansson|first4=Elizabeth|last5=Bouter|first5=Lex M.|last6=Wells|first6=George A.|last7=Hamel|first7=Candyce|last8=Shea|first8=Beverley J.|date=2009-10-01|title=AMSTAR is a reliable and valid measurement tool to assess the methodological quality of systematic reviews|url=https://www.jclinepi.com/article/S0895-4356(08)00325-9/abstract|journal=Journal of Clinical Epidemiology|language=en|volume=62|issue=10|pages=1013–1020|doi=10.1016/j.jclinepi.2008.10.009|issn=0895-4356|pmid=19230606}}</ref><ref>{{Cite journal|last=Henry|first=David A.|last2=Kristjansson|first2=Elizabeth|last3=Welch|first3=Vivian|last4=Tugwell|first4=Peter|last5=Moher|first5=David|last6=Moran|first6=Julian|last7=Hamel|first7=Candyce|last8=Thuku|first8=Micere|last9=Wells|first9=George|date=2017-09-21|title=AMSTAR 2: a critical appraisal tool for systematic reviews that include randomised or non-randomised studies of healthcare interventions, or both|url=https://www.bmj.com/content/358/bmj.j4008|journal=BMJ|language=en|volume=358|pages=j4008|doi=10.1136/bmj.j4008|issn=0959-8138|pmc=PMC5833365|pmid=28935701}}</ref> and ROBIS (Risk Of Bias In Systematic reviews); however, these are not appropriate for all systematic review types.<ref>{{Cite journal|last=Churchill|first=Rachel|last2=Kleijnen|first2=Jos|last3=Davies|first3=Philippa|last4=Shea|first4=Beverley|last5=Reeves|first5=Barnaby C.|last6=Caldwell|first6=Deborah M.|last7=Higgins|first7=Julian P. T.|last8=Savović|first8=Jelena|last9=Whiting|first9=Penny|date=2016-01-01|title=ROBIS: A new tool to assess risk of bias in systematic reviews was developed|url=https://www.jclinepi.com/article/S0895-4356(15)00308-X/abstract|journal=Journal of Clinical Epidemiology|language=en|volume=69|pages=225–234|doi=10.1016/j.jclinepi.2015.06.005|issn=0895-4356|pmc=PMC4687950|pmid=26092286}}</ref>

== About this article ==
This article is adapted from [[doi:10.15347/WJM/2020.005|a peer-reviewed version]] of this article from the [[WikiJournal of Medicine]].

=== Standardised Data on Initiatives (STARDIT) report ===
A [[wikidata:Q98550843|STARDIT]] report about this article can be found here:


* Reviewed STARDIT report version at time of publishing: [https://www.wikidata.org/w/index.php?title=Q101116128&oldid=1303112868 STARDIT Report: What are systematic reviews? (Q101116128)]
*Covidence: a paid web application and [[Cochrane (organisation)|Cochrane]] technology platform.
* 'Living' version: [[wikidata:Q101116128|STARDIT Report: What are systematic reviews? (Q101116128)]]
*DistillerSR: a paid web application
*Swift Active Screener: a paid web application
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== See also ==
== See also ==

Revision as of 06:31, 24 November 2020

Systematic reviews are a type of review that uses repeatable analytical methods to collect secondary data and analyse it. Systematic reviews are a type of evidence synthesis which formulate research questions that are broad or narrow in scope, and identify and synthesize data that directly relate to the systematic review question.[1] While some people might associate ‘systematic review’ with 'meta-analysis', there are multiple kinds of review which can be defined as ‘systematic’ which do not involve a meta-analysis. Some systematic reviews critically appraise research studies, and synthesize findings qualitatively or quantitatively.[2] Systematic reviews are often designed to provide an exhaustive summary of current evidence relevant to a research question. For example, systematic reviews of randomized controlled trials are an important way of informing evidence-based medicine,[3] and a review of existing studies is often quicker and cheaper than embarking on a new study.

While systematic reviews are often applied in the biomedical or healthcare context, they can be used in other areas where an assessment of a precisely defined subject would be helpful.[4] Systematic reviews may examine clinical tests, public health interventions, environmental interventions,[5] social interventions, adverse effects, qualitative evidence syntheses, methodological reviews, policy reviews, and economic evaluations.[6][7]

An understanding of systematic reviews and how to implement them in practice is highly recommended for professionals involved in the delivery of health care, public health and public policy. }}

Characteristics

Systematic reviews can be used to inform decision making in many different disciplines, such as evidence-based healthcare and evidence-based policy and practice.[8]

A systematic review can be designed to provide an exhaustive summary of current literature relevant to a research question.

A systematic review uses a rigorous and transparent approach for research synthesis, with the aim of assessing and, where possible, minimizing bias in the findings. While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews and other types of mixed-methods reviews which adhere to standards for gathering, analyzing and reporting evidence.[9]

Systematic reviews of quantitative data or mixed-method reviews sometimes use statistical techniques (meta-analysis) to combine results of eligible studies. Scoring levels are sometimes used to rate the quality of the evidence depending on the methodology used, although this is discouraged by the Cochrane Library.[10] As evidence rating can be subjective, multiple people may be consulted to resolve any scoring differences between how evidence is rated.[11][12][13]

The EPPI-Centre, Cochrane and the Joanna Briggs Institute have all been influential in developing methods for combining both qualitative and quantitative research in systematic reviews.[14][15][16] Several reporting guidelines exist to standardise reporting about how systematic reviews are conducted. Such reporting guidelines are not quality assessment or appraisal tools. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement[17] suggests a standardized way to ensure a transparent and complete reporting of systematic reviews, and is now required for this kind of research by more than 170 medical journals worldwide.[8] Several specialized PRISMA guideline extensions have been developed to support particular types of studies or aspects of the review process, including PRISMA-P for review protocols and PRISMA-ScR for scoping reviews.[8] A list of PRISMA guideline extensions is hosted by the EQUATOR (Enhancing the QUAlity and Transparency Of health Research) Network.[18]

For qualitative reviews, reporting guidelines include ENTREQ (Enhancing transparency in reporting the synthesis of qualitative research) for qualitative evidence syntheses; RAMESES (Realist And MEta-narrative Evidence Syntheses: Evolving Standards) for meta-narrative and realist reviews;[19][20] and eMERGe (Improving reporting of Meta-Ethnography) for meta-ethnograph.[14]

Developments in systematic reviews during the 21st century included realist reviews and the meta-narrative approach, both of which addressed problems of variation in methods and heterogeneity existing on some subjects.[21][22]

Types of systematic review

There are over 30 types of systematic review and the Table 1 below summarises some of these, but it is not exhaustive.[8][17] It is important to note that there is not always consensus on the boundaries and distinctions between the approaches described below.

Table 1: A summary of some of the types of systematic review.
Review type Summary
Mapping review/systematic map A mapping review maps existing literature and categorizes data. The method characterizes quantity and quality of literature, including by study design and other features. Mapping reviews can be used to identify the need for primary or secondary research.[8]
Meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple quantitative studies. Using statistical methods, results are combined to provide evidence from multiple studies. The two types of data generally used for meta-analysis in health research are individual participant data and aggregate data (such as odds ratios or relative risks).
Mixed studies review/mixed methods review Refers to any combination of methods where one significant stage is a literature review (often systematic). It can also refer to a combination of review approaches such as combining quantitative with qualitative research.[8]
Qualitative systematic review/qualitative evidence synthesis This method for integrates or compares findings from qualitative studies. The method can include ‘coding’ the data and looking for ‘themes’ or ‘constructs’ across studies. Multiple authors may improve the ‘validity’ of the data by potentially reducing individual bias.[8]
Rapid review An assessment of what is already known about a policy or practice issue, which uses systematic review methods to search for and critically appraise existing research. Rapid reviews are still a systematic review, however parts of the process may be simplified or omitted in order to increase rapidity.[23] Rapid reviews were used during the COVID-19 pandemic.[24]
Systematic review A systematic search for data, using a repeatable method. It includes appraising the data (for example the quality of the data) and a synthesis of research data.
Systematic search and review Combines methods from a ‘critical review’ with a comprehensive search process. This review type is usually used to address broad questions to produce the most appropriate evidence synthesis. This method may or may not include quality assessment of data sources.[8]
Systematized review Include elements of systematic review process, but searching is often not as comprehensive as a systematic review and may not include quality assessments of data sources.

Scoping reviews

Scoping reviews are distinct from systematic reviews in several important ways. A scoping review is an attempt to search for concepts by mapping the language and data which surrounds those concepts and adjusting the search method iteratively to synthesize evidence and assess the scope of an area of inquiry.[21][22] This can mean that the concept search and method (including data extraction, organisation and analysis) are refined throughout the process, sometimes requiring deviations from any protocol or original research plan.[25][26] A scoping review may often be a preliminary stage before a systematic review, which 'scopes' out an area of inquiry and maps the language and key concepts to determine if a systematic review is possible or appropriate, or to lay the groundwork for a full systematic review. The goal can be to assess how much data or evidence is available regarding a certain area of interest.[25][27] This process is further complicated if it is mapping concepts across multiple languages or cultures.

As a scoping review should be systematically conducted and reported (with a transparent and repeatable method), some academic publishers categorize them as a kind of 'systematic review', which may cause confusion. Scoping reviews are helpful when it is not possible to carry out a systematic synthesis of research findings, for example, when there are no published clinical trials in the area of inquiry. Scoping reviews are helpful when determining if it is possible or appropriate to carry out a systematic review, and are a useful method when an area of inquiry is very broad,[28] for example, exploring how the public are involved in all stages systematic reviews.[29]

There is still a lack of clarity when defining the exact method of a scoping review as it is both an iterative process and is still relatively new.[30] There have been several attempts to improve the standardisation of the method,[31][32][33][34] for example via a PRISMA guideline extension for scoping reviews (PRISMA-ScR).[35] PROSPERO (the International Prospective Register of Systematic Reviews) does not permit the submission of protocols of scoping reviews,[36] although some journals will publish protocols for scoping reviews.[29]

{{FIG}} template missing ID and not present in Wikidata.While there are multiple kinds of systematic review methods, the main stages of a review can be summarised into five stages:

1. Defining the research question

Defining an answerable question and agreeing an objective method is required to design a useful systematic review.[37] Best practice recommends publishing the protocol of the review before initiating it to reduce the risk of unplanned research duplication and to enable consistency between methodology and protocol.[38] Clinical reviews of quantitative data are often structured using the acronym PICO, which stands for 'Population or Problem', 'Intervention or Exposure', 'Comparison' and 'Outcome', with other variations existing for other kinds of research. For qualitative reviews PICo is 'Population or Problem', 'Interest' and 'Context'.

2. Searching for relevant data sources

Planning how the review will search for relevant data from research that matches certain criteria is a decisive stage in developing a rigorous systematic review. Relevant criteria can include only selecting research that is good quality and answers the defined question.[37] The search strategy should be designed to retrieve literature that matches the protocol's specified inclusion and exclusion criteria.

The methodology section of a systematic review should list all of the databases and citation indices that were searched. The titles and abstracts of identified articles can be checked against pre-determined criteria for eligibility and relevance. Each included study may be assigned an objective assessment of methodological quality, preferably by using methods conforming to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement,[18] or the high-quality standards of Cochrane.[39]

Common information sources used in searches include scholarly databases of peer-reviewed articles such as MEDLINE, Web of Science, Embase, and PubMed as well as sources of unpublished literature such as clinical trial registries and grey literature collections. Key references can also be yielded through additional methods such as citation searching, reference list checking (related to a search method called 'pearl growing'), manually searching information sources not indexed in the major electronic databases (sometimes called 'hand-searching'),[40] and directly contacting experts in the field.[41]

To be systematic, searchers must use a combination of search skills and tools such as database subject headings, keyword searching, Boolean operators, proximity searching, while attempting to balance the sensitivity (systematicity) and precision (accuracy). Inviting and involving an experienced information professional or librarian can notably improve the quality of systematic review search strategies and reporting.[42][43][44][45][46]

3. 'Extraction' of relevant data

Relevant data are 'extracted' from the data sources according to the review method. It is important to note that the data extraction method is specific to the kind of data, and data extracted on ‘outcomes’ is only relevant to certain types of reviews. For example, a systematic review of clinical trials might extract data about how the research was done (often called the method or 'intervention'), who participated in the research (including how many people), how it was paid for (for example funding sources) and what happened (the outcomes).[37] Figure 1 illustrates relevant data being extracted and 'combined' in a Cochrane intervention effect review, where a meta-analysis is possible.

4. Assess the eligibility of the data

This stage involves assessing the eligibility of data for inclusion in the review, by judging it against criteria identified at the first stage.[37] This can include assessing if a data source meets the eligibility criteria, and recording why decisions about inclusion or exclusion in the review were made. Software can be used to support the selection process including text mining tools and machine learning, which can automate aspects of the process.[47] The ‘Systematic Review Toolbox’ is a community driven, web-based catalogue of tools, to help reviewers chose appropriate tools for reviews.[48]

5. Analyse and combine the data

Analysing and combining data can provide an overall result from all the data. Because this combined result uses qualitative or quantitative data from all eligible sources of data, it is considered more reliable as it provides better evidence, as the more data included in reviews, the more confident we can be of conclusions. When appropriate, some systematic reviews include a meta-analysis, which uses statistical methods to combine data from multiple sources. A review might use quantitative data, or might employ a qualitative meta-synthesis, which synthesises data from qualitative studies. The combination of data from a meta-analysis can sometimes be visualised. One method uses a a forest plot (also called a blobbogram).[37] In an intervention effect review, the diamond in the 'forest plot' represents the combined results of all the data included.[37]

An example of a 'forest plot' is the Cochrane Collaboration logo.[37] The logo is a forest plot of one of the first reviews which showed that corticosteroids given to women who are about to give birth prematurely can save the life of the newborn child.[49]

Recent visualisation innovations include the albatross plot, which plots p-values against sample sizes, with approximate effect-size contours superimposed to facilitate analysis.[50] The contours can be used to infer effect sizes from studies that have been analysed and reported in diverse ways. Such visualisations may have advantages over other types when reviewing complex interventions.

Assessing the quality (or certainty) of evidence is an important part of some reviews. GRADE (Grading of Recommendations, Assessment, Development and Evaluations) is a transparent framework for developing and presenting summaries of evidence and is used to grade the quality of evidence.[51] The GRADE-CERQual (Confidence in the Evidence from Reviews of Qualitative research) is used to provide a transparent method for assessing the confidence of evidence from reviews or qualitative research.[52] Once these stages are complete, the review may be published, disseminated and translated into practice after being adopted as evidence.

Living systematic reviews

Living systematic reviews are a relatively new kind of high quality, semi-automated, up-to-date online summaries of research which are updated as new research becomes available.[53] The essential difference between a living systematic review and a conventional systematic review is the publication format. Living systematic reviews are 'dynamic, persistent, online-only evidence summaries, which are updated rapidly and frequently'.[54]

Research fields

Medicine and human health

History of systematic reviews in medicine

A 1904 British Medical Journal paper by Karl Pearson collated data from several studies in the UK, India and South Africa of typhoid inoculation. He used a meta-analytic approach to aggregate the outcomes of multiple clinical studies.[55] In 1972 Archie Cochrane wrote: 'It is surely a great criticism of our profession that we have not organised a critical summary, by specialty or subspecialty, adapted periodically, of all relevant randomised controlled trials'.[56] Critical appraisal and synthesis of research findings in a systematic way emerged in 1975 under the term 'meta analysis'.[57][58] Early syntheses were conducted in broad areas of public policy and social interventions, with systematic research synthesis applied to medicine and health.[59] Inspired by his own personal experiences as a senior medical officer in prisoner of war camps, Archie Cochrane worked to improve how the scientific method was used in medical evidence, writing in 1971: 'the general scientific problem with which we are primarily concerned is that of testing a hypothesis that a certain treatment alters the natural history of a disease for the better'.[60] His call for the increased use of randomised controlled trials and systematic reviews led to the creation of The Cochrane Collaboration,[61] which was founded in 1993 and named after him, building on the work by Iain Chalmers and colleagues in the area of pregnancy and childbirth.[62][56]

Current use of systematic reviews in medicine

Many organisations around the world use systematic reviews, with the methodology depending on the guidelines being followed. Organisations which use systematic reviews in medicine and human health include the National Institute for Health and Care Excellence (NICE, UK), the Agency for Healthcare Research and Quality (AHRQ, USA) and the World Health Organisation. Most notable among international organisations is Cochrane, a group of over 37,000 specialists in healthcare who systematically review randomised trials of the effects of prevention, treatments and rehabilitation as well as health systems interventions. When appropriate, they also include the results of other types of research. Cochrane Reviews are published in The Cochrane Database of Systematic Reviews section of the Cochrane Library. The 2015 impact factor for The Cochrane Database of Systematic Reviews was 6.103, and it was ranked 12th in the Medicine, General & Internal category.[63]

There are several types of Cochrane Review, including:[64][65][66][67]

  1. Intervention reviews assess the benefits and harms of interventions used in healthcare and health policy.
  2. Diagnostic test accuracy reviews assess how well a diagnostic test performs in diagnosing and detecting a particular disease.
  3. Methodology reviews address issues relevant to how systematic reviews and clinical trials are conducted and reported.
  4. Qualitative reviews synthesize qualitative evidence to address questions on aspects other than effectiveness.
  5. Prognosis reviews address the probable course or future outcome(s) of people with a health problem.
  6. Overviews of Systematic Reviews (OoRs) are a new type of study to compile multiple evidence from systematic reviews into a single document that is accessible and useful to serve as a friendly front end for the Cochrane Collaboration with regard to healthcare decision-making. These are sometimes referred to as 'umbrella reviews'.
  7. Living Systematic reviews are continually updated, incorporating relevant new evidence as it becomes available.[68] They are a relatively new kind of review, with methods still being developed and evaluated. They can be high quality, semi-automated, up-to-date online summaries of research which are updated as new research becomes available.[69] The essential difference between a 'living systematic review' and a conventional systematic review is the publication format. Living systematic reviews are 'dynamic, persistent, online-only evidence summaries, which are updated rapidly and frequently'.[70]
  8. Rapid reviews are a form of knowledge synthesis that ‘accelerates the process of conducting a traditional systematic review through streamlining or omitting specific methods to produce evidence for stakeholders in a resource-efficient manner’.[71]
  9. Reviews of complex health interventions in complex systems review interventions and interventions delivered in complex systems to improve evidence synthesis and guideline development at a global, national or health systems level.[72]

The Cochrane Collaboration provides a handbook for systematic reviewers of interventions which 'provides guidance to authors for the preparation of Cochrane Intervention reviews.'[39] The Cochrane Handbook also outlines the key steps for preparing a systematic review[39] and forms the basis of two sets of standards for the conduct and reporting of Cochrane Intervention Reviews (MECIR - Methodological Expectations of Cochrane Intervention Reviews).[73] It also contains guidance on how to undertake qualitative evidence synthesis, economic reviews and integrating patient-reported outcomes into reviews.

The Cochrane Library is a collection of databases that contains different types of independent evidence to inform healthcare decision-making. It contains a database of systematic review and meta-analyses which summarize and interpret the results of multi-disciplinary research. The library contains the Cochrane Database of Systematic Reviews (CDSR), which is a journal and database for systematic reviews in health care. The Cochrane Library also contains the Cochrane Central Register of Controlled Trials (CENTRAL) which is a database of reports of randomized and quasi-randomized controlled trials.[74] The Cochrane Library is also available in Spanish.[75]

The Cochrane Library is owned by Cochrane. It was originally published by Update Software and now published by the share-holder owned publisher John Wiley & Sons, Ltd. as part of Wiley Online Library. Royalties from sales of the Cochrane Library are the major source of funds for Cochrane (over £6 million in 2017). There are 3.66 billion people around the world who have access to the Library through national licences (national licences cost £1.5 billion[76]) or free provision for populations in low- and middle-income countries eligible under the WHO’s HINARI initiative.[76] Authors must pay an additional fee for their review to be truly open access.[77] Cochrane has an annual income of $10m USD.[78]

Public involvement and citizen science in systematic reviews

Cochrane has several tasks that the public or other 'stakeholders' can be involved in doing, associated with producing systematic reviews and other outputs. Tasks can be organised as 'entry level' or higher. Tasks include:

  • Joining a collaborative volunteer effort to help categorise and summarise healthcare evidence[79]
  • Data extraction and risk of bias assessment
  • Translation of reviews into other languages

A recent systematic review of how people were involved in systematic reviews aimed to document the evidence-base relating to stakeholder involvement in systematic reviews and to use this evidence to describe how stakeholders have been involved in systematic reviews.[80] Thirty percent involved patients and/or carers. The ACTIVE framework provides a way to consistently describe how people are involved in systematic review, and may be used as a way to support the decision-making of systematic review authors in planning how to involve people in future reviews.[81] Standardised Data on Initiatives (STARDIT) is another proposed way of reporting who has been involved in which tasks during research, including systematic reviews.[82]

While there has been some criticism of how Cochrane prioritises systematic reviews,[83] a recent project involved people in helping identify research priorities to inform future Cochrane Reviews.[84][85] In 2014, the Cochrane-Wikipedia partnership was formalised. This supports the inclusion of relevant evidence within all Wikipedia medical articles, as well as other processes to help ensure that medical information included in Wikipedia is of the highest quality and accuracy.[86]

Learning resources

Cochrane has produced many learning resources to help people understand what systematic reviews are, and how to do them. Most of the learning resources can be found at the 'Cochrane Training' webpage,[87] which also includes a link to the book Testing Treatments, which has been translated into many languages.[88] In addition, Cochrane has created a short video What are Systematic Reviews which explains in plain English how they work and what they are used for.[89] The video has been translated into multiple languages,[90] and viewed over 192,282 times (as of August 2020). In addition, an animated storyboard version was produced and all the video resources were released in multiple versions under Creative Commons for others to use and adapt.[91][92][93][94] The Critical Appraisal Skills Programme (CASP) provides free learning resources to support people to appraise research critically, including a checklist which contains 10 questions to 'help you make sense of a systematic review'.[95][96]

Social, behavioural and educational

Several organisations use systematic reviews in social, behavioural, and educational areas of evidence-based policy, including the National Institute for Health and Care Excellence (NICE, UK), Social Care Institute for Excellence (SCIE, UK), the Agency for Healthcare Research and Quality (AHRQ, USA), the World Health Organisation, the International Initiative for Impact Evaluation (3ie), the Joanna Briggs Institute and the Campbell Collaboration. The quasi-standard for systematic review in the social sciences is based on the procedures proposed by the Campbell Collaboration, which is one of several groups promoting evidence-based policy in the social sciences. The Campbell Collaboration: 'helps people make well-informed decisions by preparing, maintaining and disseminating systematic reviews in education, crime and justice, social welfare and international development.'[97] The Campbell Collaboration is a sibling initiative of Cochrane, and was created in 2000 at the inaugural meeting in Philadelphia, USA, attracting 85 participants from 13 countries.[98]

Business and economics

Due to the different nature of research fields outside of the natural sciences, the aforementioned methodological steps cannot easily be applied in all areas of business research. Some attempts to transfer the procedures from medicine to business research have been made,[99] including a step-by-step approach,[100] and developing a standard procedure for conducting systematic literature reviews in business and economics. The Campbell & Cochrane Economics Methods Group (C-CEMG) works to improve the inclusion of economic evidence into Cochrane and Campbell systematic reviews of interventions, to enhance the usefulness of review findings as a component for decision-making.[101] Such economic evidence is crucial for health technology assessment processes.

International development research

Systematic reviews are increasingly prevalent in other fields, such as international development research.[102] Subsequently, several donors (including the UK Department for International Development (DFID) and AusAid) are focusing more attention and resources on testing the appropriateness of systematic reviews in assessing the impacts of development and humanitarian interventions.[102]

Environment

The Collaboration for Environmental Evidence (CEE) works to achieve a sustainable global environment and the conservation of biodiversity. The CEE has a journal titled Environmental Evidence which publishes systematic reviews, review protocols and systematic maps on impacts of human activity and the effectiveness of management interventions.[103]

Review tools

A 2019 publication identified 15 systematic review tools and ranked them according to the number of 'critical features' as required to perform a systematic review, including:[104]

  • DistillerSR: a paid web application
  • Swift Active Screener: a paid web application
  • Covidence: a paid web application and Cochrane technology platform.
  • Rayyan: a free web application
  • Sysrev: a free web application

Limitations

Out-dated or risk of bias

While systematic reviews are regarded as the strongest form of evidence, a 2003 review of 300 studies found that not all systematic reviews were equally reliable, and that their reporting can be improved by a universally agreed upon set of standards and guidelines.[105] A further study by the same group found that of 100 systematic reviews monitored, 7% needed updating at the time of publication, another 4% within a year, and another 11% within 2 years; this figure was higher in rapidly changing fields of medicine, especially cardiovascular medicine.[106] A 2003 study suggested that extending searches beyond major databases, perhaps into grey literature, would increase the effectiveness of reviews.[107]

Some authors have highlighted problems with systematic reviews, particularly those conducted by Cochrane, noting that published reviews are often biased, out of date and excessively long.[108] Cochrane reviews have been criticized as not being sufficiently critical in the selection of trials and including too many of low quality. They proposed several solutions, including limiting studies in meta-analyses and reviews to registered clinical trials, requiring that original data be made available for statistical checking, paying greater attention to sample size estimates, and eliminating dependence on only published data.

Some of these difficulties were noted as early as 1994:

much poor research arises because researchers feel compelled for career reasons to carry out research that they are ill equipped to perform, and nobody stops them.

DG Altman, 1994[109]

Methodological limitations of meta-analysis have also been noted.[110] Another concern is that the methods used to conduct a systematic review are sometimes changed once researchers see the available trials they are going to include.[111] Some website have described retractions of systematic reviews and published reports of studies included in published systematic reviews.[112][113][114] Eligibility criteria must be justifiable and not arbitrary (for example, the date range searched) as this may affect the perceived quality of the review.[115][116]

Limited reporting of clinical trials and data from human studies

The 'AllTrials' campaign highlights that around half of clinical trials have never reported results and works to improve reporting.[117] This lack of reporting has extremely serious implications for research, including systematic reviews, as it is only possible to synthesize data of published studies. In addition, 'positive' trials were twice as likely to be published as those with 'negative' results.[118] At present, it is legal for for-profit companies to conduct clinical trials and not publish the results.[119] For example, in the past 10 years 8.7 million patients have taken part in trials that have not published results.[119] These factors mean that it is likely there is a significant publication bias, with only 'positive' or perceived favourable results being published. A recent systematic review of industry sponsorship and research outcomes concluded that 'sponsorship of drug and device studies by the manufacturing company leads to more favorable efficacy results and conclusions than sponsorship by other sources' and that the existence of an industry bias that cannot be explained by standard 'Risk of bias' assessments.[120] Systematic reviews of such a bias may amplify the effect, although it is important to note that the flaw is in the reporting of research generally, not in the systematic review method.

Poor compliance with review reporting guidelines

The rapid growth of systematic reviews in recent years has been accompanied by the attendant issue of poor compliance with guidelines, particularly in areas such as declaration of registered study protocols, funding source declaration, risk of bias data, and description of clear study objectives.[121][122][123][124] A host of studies have identified weaknesses in the rigour and reproducibility of search strategies in systematic reviews.[125][126][127][128][129][130] To remedy this issue, a new PRISMA guideline extension called PRISMA-S is being developed to improve the quality, reporting, and reproducibility of systematic review search strategies.[131][132] Furthermore, tools and checklists for peer-reviewing search strategies have been created, such as the Peer Review of Electronic Search Strategies (PRESS) guidelines.[133]

A key challenge for using systematic reviews in clinical practice and healthcare policy is assessing the quality of a given review. Consequently, a range of appraisal tools to evaluate systematic reviews have been designed. The two most popular measurement instruments and scoring tools for systematic review quality assessment are AMSTAR 2 (a measurement tool to assess the methodological quality of systematic reviews)[134][135][136][137] and ROBIS (Risk Of Bias In Systematic reviews); however, these are not appropriate for all systematic review types.[138]

About this article

This article is adapted from a peer-reviewed version of this article from the WikiJournal of Medicine.

Standardised Data on Initiatives (STARDIT) report

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See also

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