To discuss particular sources, see the reliable sources noticeboard. For vandalism, see WP:VAND.

The threshold for inclusion in Wikipedia is verifiability, not truth—whether readers can check that material in Wikipedia has already been published by a reliable source, not whether editors think it is true.

All material in Wikipedia articles must be attributable to a reliable published source to show that it is not original research, but in practice not everything need actually be attributed. This policy requires that anything challenged or likely to be challenged, including all quotations, be attributed to a reliable source in the form of an inline citation, and that the source directly supports the material in question. See the discussion on sources in WP:NOR that describes summarizing matters fully and clearly in your own words, leaving nothing implied that goes beyond the sources. For how to write citations, see Citing sources. This verifiability policy is strictly applied to all material in the mainspace—articles, lists, sections of articles, and captions—without exception, and in particular to material about living persons. Anything that requires but lacks a source may be removed, and unsourced contentious material about living persons must be removed immediately.

Verifiability is one of Wikipedia's core content policies, along with No original research and Neutral point of view. Jointly, these determine the type and quality of material that is acceptable in articles. They should not be interpreted in isolation from one another, and editors should therefore familiarize themselves with all three.

For how to write citations, see Wikipedia:Citing sources

The burden of evidence lies with the editor who adds or restores the material. All quotations and any material challenged or likely to be challenged must be attributed to a reliable, published source using an inline citation. The source should be cited clearly and precisely, with page numbers where appropriate, and must clearly support the material as presented in the article. Drawing inferences from multiple sources to advance a novel position is prohibited by the no original research policy.^[1] If no reliable third-party sources can be found on a topic, Wikipedia should not have an article on it.

Any material lacking a reliable source directly supporting it may be removed. How quickly this should happen depends on the material and the overall state of the article. Editors might object if you remove material without giving them time to provide references. It has always been good practice to make reasonable efforts to find sources yourself that support such material, and cite them. Do not leave unsourced or poorly sourced material in an article if it might damage the reputation of living persons or organizations, and do not move it to the talk page.^[2]

All articles must adhere to Wikipedia's neutrality policy, fairly representing all majority and significant-minority viewpoints that have been published by reliable sources, in rough proportion to the prominence of each view. Tiny-minority views need not be included, except in articles devoted to them. Where there is disagreement between sources, their views should be clearly attributed in the text: "John Smith argues that X, while Paul Jones maintains that Y," followed by an inline citation.

Reliable sources

The word "source" as used in Wikipedia has three meanings: the piece of work itself (a document, article, paper, or book), the creator of the work (for example, the writer), and the publisher of the work (for example, The New York Times). All three can affect reliability.

Articles should be based on reliable, third-party, published sources with a reputation for fact-checking and accuracy. Citing these sources prevents unverifiable claims from being added to articles, and makes it easier to identify plagiarism and copyright violations. Sources should directly support the material as it is presented in an article, and should be appropriate to the claims made.

The appropriateness of any source depends on the context. In general, the best sources have a professional structure in place for checking or analyzing scientific findings, evidence, facts, and legal aspects; as a rule of thumb, the greater the degree of scrutiny given to these issues, the more reliable the source.

Academic publications, such as peer-reviewed journals and books published by well-regarded academic presses, are usually the most reliable sources where available. Non-academic sources can be used, particularly if they are respected mainstream publications. This includes books by reputable publishers as well as newspapers, magazines, journals and electronic media.

In topics which are the subject of scholarly research, the most authoritative sources are usually academic works that have undergone scrutiny by a community of experts in a field, particularly peer-reviewed systematic reviews. Quality mainstream media sources can be used for areas such as current affairs – including the socio-economic, political, and human impact of science – or biographies of living persons. Non-academic sources may misreport or misinterpret data and its significance, and should therefore not be relied upon exclusively as sources of that kind of material where academic secondary sources are available.

Self-published expert sources are considered reliable in defined circumstances (see below). All self-published sources, whether experts or not, are considered reliable as sources on themselves, especially in articles about themselves, subject to certain criteria, though no article should be based primarily on such sources (see below).

Newspaper and magazine blogs

Personal and group blogs are largely not acceptable as sources; see below. Some newspapers host interactive columns that they call blogs; these are acceptable as sources so long as the writers are professionals and the blog is subject to the newspaper's full editorial control. In March 2010, the Press Complaints Commission in the UK ruled that journalists' blogs hosted on the websites of newspapers or magazines are subject to the same standards expected of comment pieces in that organization's print editions.^[3] Where a news organization publishes an opinion piece, the writer should be attributed (e.g. "Jane Smith has suggested..."). Posts left by readers may never be used as sources.

Questionable sources

Questionable sources are those with a poor reputation for checking the facts, or with no editorial oversight. Such sources include websites and publications expressing views that are widely acknowledged as extremist, or promotional, or which rely heavily on rumor and personal opinion. Questionable sources should be used only as sources of material on themselves, especially in articles about themselves; see below. Questionable sources are generally unsuitable for citing contentious claims about third parties.

Self-published sources (online and paper)

Anyone can create a website or pay to have a book published, then claim to be an expert in a certain field. For that reason self-published media—including but not limited to books, newsletters, personal websites, open wikis, personal or group blogs, Internet forum postings, and tweets—are largely not acceptable.

Self-published material may be acceptable when produced by an established expert on the topic of the article whose work in the relevant field has previously been published by reliable third-party publications. Caution should be exercised when using such sources: if the information in question is really worth reporting, someone else is likely to have done so. Self-published sources should never be used as third-party sources about living persons, even if the author is a well-known professional researcher or writer: see WP:BLP#Reliable sources.

Self-published sources as sources on themselves

Self-published sources may be used as sources of information about themselves, especially in articles about themselves, without the requirement that they be published experts in the field, so long as:

1. the material is not unduly self-serving;
2. it does not involve claims about third parties;
3. it does not involve claims about events not directly related to the subject;
4. there is no reasonable doubt as to its authenticity;
5. the article is not based primarily on such sources.

Wikipedia and sources that mirror or use it

Articles on Wikipedia, or on websites that mirror its content, should not be used as sources, because this would amount to self-reference. Similarly, editors should not use sources that present material originating from Wikipedia to support that same material in Wikipedia, as this would create circular sourcing—Wikipedia citing a source that derives its material from Wikipedia. Wikipedia may be cited with caution as a primary source of information on itself, such as in articles about itself.

Non-English sources

Because this is the English Wikipedia, English-language sources are preferred over non-English ones, unless no English sources of equal quality and relevance are available. When quoting a source in a different language, provide both the original-language text and an English translation in the text or a footnote. Translations published by reliable sources are preferred over translations by Wikipedians. When citing such a source without quoting it, the original and its translation should be provided if requested by other editors: this can be added to a footnote or the talk page. When posting original source material, editors should be careful not to violate copyright; see the fair-use guideline.

Certain red flags should prompt editors to examine the sources for a given claim:

• Surprising or apparently important claims not covered by mainstream sources;
• Reports of a statement by someone that seems out of character, embarrassing, controversial, or against an interest they had previously defended;
• Claims that are contradicted by the prevailing view within the relevant community, or that would significantly alter mainstream assumptions, especially in science, medicine, history, politics, and biographies of living persons. This is especially true when proponents say there is a conspiracy to silence them.

Exceptional claims in Wikipedia require high-quality sources.^[4] If such sources are not available, the material should not be included. Also be sure to adhere to other policies, such as the policy for biographies of living persons and the undue weight provision of Wikipedia:Neutral point of view.

Verifiability in this context means that anyone should be able to check that material in a Wikipedia article has already been published by a reliable source, as required by this policy and by No original research. The principle of verifiability implies nothing about ease of access to sources: some online sources may require payment, while some print sources may be available only in university libraries. WikiProject Resource Exchange may be able to assist in obtaining material that is not easily accessible.

To discuss the reliability of a specific source for a particular statement, consult the reliable sources noticeboard, which seeks to apply this policy to particular cases. For a guideline discussing the reliability of particular types of sources, see Wikipedia:Identifying reliable sources (WP:IRS). In the case of inconsistency between this policy and the WP:IRS guideline, or any other guideline, the policy has priority.

If you want to request a source for an unsourced statement, consider tagging a sentence with the {{citation needed}} template by writing {{cn}} or {{fact}}. Other templates are available here for tagging sections or entire articles. Alternatively, leave a note on the talk page requesting a source, or move the material there. To request verification that a reference supports the text, tag it with {{verification needed}}. Material that fails verification may be tagged with {{failed verification}} or removed. Unsourced or poorly sourced contentious material about living persons should be removed immediately and not tagged or moved to the talk page.

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• Wikipedia:Citing sources, a guideline
• Wikipedia:Core content policies, an essay with a summary of these policies and their brief history
• Wikipedia:Fringe theories, a guideline
• Wikipedia:Identifying reliable sources (medicine), a guideline
• Wikipedia:List of free online resources
• Wikipedia:Template messages/Sources of articles
• WikiProject Fact and Reference Check
• WikiProject Resource Exchange
• Wikipedia:When to cite, an essay