Wikipedia articles should be based on reliable, published sources, making sure that all majority and significant minority views that have appeared in those sources are covered (see Wikipedia:Neutral point of view). If no reliable sources can be found on a topic, Wikipedia should not have an article on it.
This guideline discusses the reliability of various types of sources. The policy on sourcing is Wikipedia:Verifiability, which requires inline citations for any material challenged or likely to be challenged, and for all quotations. The verifiability policy is strictly applied to all material in the mainspace—articles, lists, and sections of articles—without exception, and in particular to biographies of living persons, which states:
Contentious material about living persons (or, in some cases, recently deceased) that is unsourced or poorly sourced—whether the material is negative, positive, neutral, or just questionable—should be removed immediately and without waiting for discussion.
In the event of a contradiction between this guideline and our policies regarding sourcing and attribution, the policies take priority and editors should seek to resolve the discrepancy. Other policies relevant to sourcing are Wikipedia:No original research and Wikipedia:Biographies of living persons. For questions about the reliability of particular sources, see Wikipedia:Reliable sources/Noticeboard.
Articles should be based on reliable, independent, published sources with a reputation for fact-checking and accuracy. This means that we publish the opinions only of reliable authors, and not the opinions of Wikipedians who have read and interpreted primary source material for themselves. The following examples cover only some of the possible types of reliable sources and source reliability issues, and are not intended to be exhaustive. Proper sourcing always depends on context; common sense and editorial judgment are an indispensable part of the process.
The word "source" when citing sources on Wikipedia has three related meanings:
Any of the three can affect reliability. Reliable sources may be published materials with a reliable publication process, authors who are regarded as authoritative in relation to the subject, or both. These qualifications should be demonstrable to other people.
The term "published" is most commonly associated with text materials, either in traditional printed format or online; however, audio, video, and multimedia materials that have been recorded then broadcast, distributed, or archived by a reputable party may also meet the necessary criteria to be considered reliable sources. Like text, media must be produced by a reliable source and be properly cited. Additionally, an archived copy of the media must exist. It is convenient, but by no means necessary, for the archived copy to be accessible via the Internet.
The reliability of a source depends on context. Each source must be carefully weighed to judge whether it is reliable for the statement being made in the Wikipedia article and is an appropriate source for that content. In general, the more people engaged in checking facts, analyzing legal issues, and scrutinizing the writing, the more reliable the publication. Information provided in passing by an otherwise reliable source that is not related to the principal topics of the publication may not be reliable; editors should cite sources focused on the topic at hand where possible. Sources should directly support the information as it is presented in the Wikipedia article.
Especially in scientific and academic fields, older sources may be inaccurate because new information has been brought to light, new theories proposed, or vocabulary changed. In areas like politics or fashion, laws or trends may make older claims incorrect. Be sure to check that older sources have not been superseded, especially if it is likely the new discoveries or developments have occurred in the last few years. In particular, newer sources are generally preferred in medicine.
Sometimes sources are too new to use, such as with breaking news (where later reports might be more accurate), and primary sources which purport to debunk a long-standing consensus or introduce a new discovery (in which case awaiting studies that attempt to replicate the discovery might be a good idea, or reviews that validate the methods used to make the discovery).
With regard to historical events, older reports (closer to the event, but not too close such that they are prone to the errors of breaking news) tend to have the most detail, and are less likely to have errors introduced by repeated copying and summarizing. However, newer secondary and tertiary sources may have done a better job of collecting more reports from primary sources and resolving conflicts, applying modern knowledge to correctly explain things that older sources could not have, or remaining free of bias that might affect sources written while any conflicts described were still active or strongly felt.
Sources of any age may be prone to recentism, and this needs to be balanced out by careful editing.
Many Wikipedia articles rely on scholarly material. When available, academic and peer-reviewed publications, scholarly monographs, and textbooks are usually the most reliable sources. However, some scholarly material may be outdated, in competition with alternative theories, or controversial within the relevant field. Try to cite current scholarly consensus when available, recognizing that this is often absent. Reliable non-academic sources may also be used in articles about scholarly issues, particularly material from high-quality mainstream publications. Deciding which sources are appropriate depends on context. Material should be attributed in-text where sources disagree.
News sources often contain both factual content and opinion content. News reporting from well-established news outlets is generally considered to be reliable for statements of fact (though even the most reputable reporting sometimes contains errors). News reporting from less-established outlets is generally considered less reliable for statements of fact. Most newspapers also reprint items from news agencies such as Reuters, Interfax, Agence France-Presse, United Press International or the Associated Press, which are responsible for accuracy. The agency should be cited in addition to the newspaper that reprinted it.
Editorial commentary, analysis and opinion pieces, whether written by the editors of the publication (editorials) or outside authors (op-eds) are reliable primary sources for statements attributed to that editor or author, but are rarely reliable for statements of fact. Human interest reporting is generally not as reliable as news reporting, and may not be subject to the same rigorous standards of fact-checking and accuracy (see junk food news).
Although the content guidelines for external links prohibit linking to "Individual web pages that primarily exist to sell products or services," inline citations may be allowed to e-commerce pages such as that of a book on a bookseller's page or an album on its streaming-music page, in order to verify such things as titles and running times. Journalistic and academic sources are preferable, however, and e-commerce links should be replaced with reliable non-commercial sources if available.
Rankings proposed by vendors (such as bestseller lists at Amazon) usually have at least one of the following problems:
For such reasons, such rankings are usually avoided as Wikipedia content.
Wikipedia articles are required to present a neutral point of view. However, reliable sources are not required to be neutral, unbiased, or objective. Sometimes non-neutral sources are the best possible sources for supporting information about the different viewpoints held on a subject.
Common sources of bias include political, financial, religious, philosophical, or other beliefs. Although a source may be biased, it may be reliable in the specific context. When dealing with a potentially biased source, editors should consider whether the source meets the normal requirements for reliable sources, such as editorial control, a reputation for fact-checking, and the level of independence from the topic the source is covering. Bias may make in-text attribution appropriate, as in "Feminist Betty Friedan wrote that..."; "According to the Marxist economist Harry Magdoff..."; or "Conservative Republican presidential candidate Barry Goldwater believed that...".
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, that are promotional in nature, or that rely heavily on rumors and personal opinions. Questionable sources are generally unsuitable for citing contentious claims about third parties, which includes claims against institutions, persons living or dead, as well as more ill-defined entities. The proper uses of a questionable source are very limited.
Beware of sources that sound reliable but do not have the reputation for fact-checking and accuracy that this guideline requires. The Journal of 100% Reliable Factual Information might have a reputation for "predatory" behavior, which includes questionable business practices and/or peer-review processes that raise concerns about the reliability of their journal articles.
Sponsored content is generally unacceptable as a source, because it is paid for by advertisers and bypasses the publication's editorial process. Reliable publications clearly indicate sponsored articles in the byline or with a disclaimer at the top of the article. Sources that do not clearly distinguish staff-written articles from sponsored content are also questionable.
Symposia and supplements to academic journals are often (but far from always) unacceptable sources. They are commonly sponsored by industry groups with a financial interest in the outcome of the research reported. They may lack independent editorial oversight and peer review, with no supervision of content by the parent journal. Such shill articles do not share the reliability of their parent journal, being essentially paid ads disguised as academic articles. Such supplements, and those that do not clearly declare their editorial policy and conflicts of interest, should not be cited.
Indications that an article was published in a supplement may be fairly subtle; for instance, a letter "S" added to a page number, or "Suppl." in a reference. However, note that merely being published in a supplement is not prima facie evidence of being published in a sponsored supplement. Many, if not most, supplements are perfectly legitimate sources, such as the Astronomy & Astrophysics Supplement Series, Nuclear Physics B: Proceedings Supplements, Supplement to the London Gazette, or The Times Higher Education Supplement. A sponsored supplement also does not necessarily involve a COI; for instance, public health agencies may also sponsor supplements. However, groups that do have a COI may hide behind layers of front organizations with innocuous names, so the ultimate funding sources should always be ascertained.
Anyone can create a personal web page or publish their own book and claim to be an expert in a certain field. For that reason, self-published sources are largely not acceptable. Self-published books and newsletters, personal pages on social networking sites, tweets, and posts on Internet forums are all examples of self-published media. Self-published expert sources may be considered reliable when produced by an established expert on the subject matter, whose work in the relevant field has previously been published by reliable, independent publications. Never use self-published sources as independent sources about living people, even if the author is an expert, well-known professional researcher, or writer.
Content from websites whose content is largely user-generated is generally unacceptable. Sites with user-generated content include personal websites, personal and group blogs (excluding newspaper and magazine blogs), content farms, Internet forums, social media sites, video and image hosting services, most wikis, and other collaboratively created websites.
Self-published or questionable 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 the following criteria are met:
These requirements also apply to pages from social networking websites such as Twitter, Tumblr, and Facebook. Use of self-sourced material should be de minimis; the great majority of any article must be drawn from independent sources.
Editors must take particular care when writing biographical material about living persons. Contentious material about a living person that is unsourced or poorly sourced should be removed immediately; do not move it to the talk page. This applies to any material related to living persons on any page in any namespace, not just article space.
Wikipedia articles should be based mainly on reliable secondary sources, i.e., a document or recording that relates or discusses information originally presented elsewhere.
Reputable tertiary sources, such as introductory-level university textbooks, almanacs, and encyclopedias, may be cited. However, although Wikipedia articles are tertiary sources, Wikipedia employs no systematic mechanism for fact checking or accuracy. Thus, (except as sources on themselves per WP:SELFSOURCE).Wikipedia articles (and Wikipedia mirrors) in themselves are not reliable sources for any purpose
Primary sources are often difficult to use appropriately. Although they can be both reliable and useful in certain situations, they must be used with caution in order to avoid original research. Although specific facts may be taken from primary sources, secondary sources that present the same material are preferred. Large blocks of material based purely on primary sources should be avoided. All interpretive claims, analyses, or synthetic claims about primary sources must be referenced to a secondary source, rather than original analysis of the primary-source material by Wikipedia editors.
Ideal sources for biomedical assertions include general or systematic reviews in reliable, independent, published sources, such as reputable medical journals, widely recognised standard textbooks written by experts in a field, or medical guidelines and position statements from nationally or internationally reputable expert bodies. It is vital that the biomedical information in all types of articles be based on reliable, independent, published sources and accurately reflect current medical knowledge.
The accuracy of quoted material is paramount and the accuracy of quotations from living persons is especially sensitive. To ensure accuracy, the text of quoted material is best taken from (and cited to) the original source being quoted. If this is not possible, then the text may be taken from a reliable secondary source (ideally one that includes a citation to the original). No matter where you take the quoted text from, it is important to .
Partisan secondary sources should be viewed with suspicion as they may misquote or quote out of context. In such cases, look for neutral corroboration from another source.
Any analysis or interpretation of the quoted material, however, should rely on a secondary source (see Wikipedia:No original research).
A statement that all or most scientists or scholars hold a certain view requires reliable sourcing that directly says that all or most scientists or scholars hold that view. Otherwise, individual opinions should be identified as those of particular, named sources. Editors should avoid original research especially with regard to making blanket statements based on novel syntheses of disparate material. Stated simply, any statement in Wikipedia that academic consensus exists on a topic must be sourced rather than being based on the opinion or assessment of editors. Review articles, especially those printed in academic review journals that survey the literature, can help clarify academic consensus.
How accepted, high-quality reliable sources use a given source provides evidence, positive or negative, for its reliability and reputation. The more widespread and consistent this use is, the stronger the evidence. For example, widespread citation without comment for facts is evidence of a source's reputation and reliability for similar facts, whereas widespread doubts about reliability weigh against it. If outside citation is the main indicator of reliability, particular care should be taken to adhere to other guidelines and policies, and to not represent unduly contentious or minority claims. The goal is to reflect established views of sources as far as we can determine them.
Some sources may be considered reliable for statements as to their author's opinion, but not for statements asserted as fact. For example, an inline qualifier might say "[Author XYZ] says....". A prime example of this is opinion pieces in sources recognized as reliable. When using them, it is best to clearly attribute the opinions in the text to the author and make it clear to the reader that they are reading an opinion.
Otherwise reliable news sources—for example, the website of a major news organization—that publish in a blog-style format for some or all of their content may be as reliable as if published in standard news article format.
There is an important exception to sourcing statements of fact or opinion: Never use self-published books, zines, websites, webforums, blogs and tweets as a source for material about a living person, unless written or published by the subject of the biographical material. "Self-published blogs" in this context refers to personal and group blogs; see and .
Breaking-news reports often contain serious inaccuracies. As an electronic publication, Wikipedia can and should be up to date, but Wikipedia is not a newspaper and it does not need to go into all details of a current event in real time. It is better to wait a day or two after an event before adding details to the encyclopedia, than to help spread potentially false rumors. This gives journalists time to collect more information and verify claims, and for investigative authorities to make official announcements. The On the Media Breaking News Consumer's Handbook contains several suggestions to avoid spreading unreliable and false information, such as distrusting anonymous sources and unconfirmed reports, as well as reports attributed to other news media; seeking multiple sources; seeking eyewitness reports; being wary of potential hoaxes, and being skeptical of reports of possible additional attackers in mass shootings.
Claims sourced to initial news reports should be immediately replaced with better-researched ones as soon as they are published, especially if those original reports contained inaccuracies. All breaking-news stories, without exception, are primary sources, and must be treated with caution: see .
A small number of sources are deprecated on Wikipedia. That means they should not be used, unless there is a specific consensus to do so. Deprecation happens through a request for comment, usually at the reliable sources noticeboard. It is reserved for sources that have a substantial history of fabrication or other serious factual accuracy issues (e.g. promoting conspiracy theories), usually when there are large numbers of references to the source giving rise to concerns about the integrity of information in the encyclopaedia.
A deprecated source should not be used to support factual claims. While there are exceptions for discussion of the source's own view on something, these are rarely appropriate outside articles on the source itself. In general articles, commentary on a deprecated source's opinion should be drawn from independent secondary sources. Including a claim or statement by a deprecated source that is not covered by reliable sources risks giving undue weight to a fringe view.
Some sources are blacklisted, and can not be used at all. Blacklisting is generally reserved for sources which are added abusively, such as state-sponsored fake news sites with a history of addition by troll farms. Specific blacklisted sources can be locally whitelisted; see Wikipedia:Blacklist for other details about blacklisting.