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Source: Wikipedia, the free encyclopedia.

In the English Wikipedia, verifiability means other people using the encyclopedia can check that the information comes from a reliable source. Its content is determined by previously published information rather than editors' beliefs, opinions, experiences, or previously unpublished ideas or information. Even if you are sure something is true, it must have been previously published in a reliable source before you can add it.[a] If reliable sources disagree with each other, then maintain a neutral point of view and present what the various sources say, giving each side its due weight.

All material in

inline citation to a reliable source that directly supports[b]
the material. The four types are:

Any material that needs an inline citation but does not have one may be removed. Please immediately remove contentious material about living people that is unsourced or poorly sourced.

For how to write citations, see citing sources. Verifiability, no original research, and neutral point of view are Wikipedia's core content policies. They work together to determine content, so editors should understand the key points of all three. Articles must also comply with the copyright policy.

Responsibility for providing citations

All content must be verifiable. The burden to demonstrate verifiability lies with the editor who adds or restores material, and it is satisfied by providing an inline citation to a reliable source that directly supports[b] the contribution.[c]

Using inline citations, provide reliable, published sources for:

The cited source must clearly support the material as presented in the article. Cite the source clearly, ideally giving page number(s)—though sometimes a section, chapter, or other division may be appropriate instead; see Wikipedia:Citing sources for details of how to do this.

Any material lacking an inline citation to a reliable source that directly supports

you are encouraged to provide an inline citation yourself
before considering whether to remove or tag it.

Do not leave unsourced or poorly sourced material in an article if it might damage the reputation of living people[1] or existing groups, and do not move it to the talk page. You should also be aware of how Wikipedia:Biographies of living persons also applies to groups.

Reliable sources

What counts as a reliable source

A cited source on Wikipedia is often a specific portion of text (such as a short article or a page in a book). But when editors discuss sources (for example, to debate their appropriateness or reliability) the word source has four related meanings:

  • The work itself (the article, book: "That book looks like a useful source for this article.") and works like it ("An obituary can be a useful biographical source", "A recent source is better than an old one")
  • The creator of the work (the writer, journalist: "What do we know about that source's reputation?") and people like them ("A medical researcher is a better source than a journalist for medical claims").
  • The publication (for example, the newspaper, journal, magazine: "That source covers the arts.") and publications like them ("A newspaper is not a reliable source for medical claims").
  • The publisher of the work (for example, Cambridge University Press: "That source publishes reference works.") and publishers like them ("An academic publisher is a good source of reference works").

All four can affect reliability.

Base articles on reliable, independent, published sources with a reputation for fact-checking and accuracy. Source material must have been published, the definition of which for the purposes of Wikipedia is made available to the public in some form.[f] Unpublished materials are not considered reliable. Use sources that directly support the material presented in an article and are appropriate to the claims made. The appropriateness of any source depends on the context. Be especially careful when sourcing content related to living people or medicine.

If available, academic and

publications are usually the most reliable sources on topics such as history, medicine, and science.

Editors may also use material from reliable non-academic sources, particularly if it appears in respected

publications. Other reliable sources include:

  • University-level textbooks
  • Books published by respected
    publishing houses
  • Mainstream (
    ) magazines, including specialty ones
  • Reputable newspapers

Editors may also use electronic media, subject to the same criteria (see details in

Wikipedia:Identifying reliable sources and Wikipedia:Search engine test

Best sources


best sources
have a professional structure for checking or analyzing facts, legal issues, evidence, and arguments. The greater the degree of scrutiny given to these issues, the more reliable the source.

Newspaper and magazine blogs

Some newspapers, magazines, and other news organizations host online

opinion piece in a blog, attribute the statement to the writer, e.g. "Jane Smith wrote ..." Never use the blog comments that are left by the readers as sources. For personal or group blogs that are not reliable sources, see § Self-published sources

Reliable sources noticeboard and guideline

To discuss the reliability of a specific source for a particular statement, consult Wikipedia: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:Reliable sources. In the case of inconsistency between this policy and the Wikipedia:Reliable sources guideline, or any other guideline related to sourcing, this policy has priority.

Sources that are usually not reliable

Questionable sources

Questionable sources are those that have a poor reputation for checking the facts, lack meaningful editorial oversight, or

have an apparent conflict of interest

Such sources include websites and publications expressing views widely considered by other sources to be promotional, extremist, or relying heavily on unsubstantiated gossip, rumor, or personal opinion. Questionable sources should be used only as sources for material on themselves, such as in articles about themselves; see below. They are not suitable sources for contentious claims about others.

Predatory open access
journals are considered questionable due to the absence of quality control in the peer-review process.

Self-published sources

Anyone can create a

third-party sources
about living people, even if the author is an expert, well-known professional researcher, or writer.

Self-published or questionable sources as sources on themselves

Self-published and questionable sources may be used as sources of information about themselves, usually in articles about themselves or their activities, without the self-published source requirement that they are established experts in the field, so long as:

  1. The material is neither unduly self-serving nor an exceptional claim;
  2. It does not involve claims about third parties;
  3. It does not involve claims about events not directly related to the source;
  4. There is no reasonable doubt as to its authenticity; and
  5. The article is not based primarily on such sources.

This policy also applies to material made public by the source on social networking websites such as Twitter, Tumblr, LinkedIn, Reddit, and Facebook.

Wikipedia and sources that mirror or use it

Do not use articles from Wikipedia (whether English Wikipedia or Wikipedias in other languages) as sources, since Wikipedia is a

reliable sources. Confirm that these sources support the content, then use them directly.[3]

An exception is allowed when Wikipedia itself is being discussed in the article. These may cite an article, guideline, discussion, statistic, or other content from Wikipedia (or a sister project) to support a statement about Wikipedia. Wikipedia or the sister project is a

undue emphasis on Wikipedia's role or views, and inappropriate self-reference
. The article text should clarify how the material is sourced from Wikipedia to inform the reader about the potential bias.


Access to sources

Do not reject reliable sources just because they are difficult or costly to access. Some reliable sources are not easily accessible. For example, an online source may require payment, and a print-only source may be available only through libraries. Rare historical sources may even be available only in special museum collections and archives. If you have trouble accessing a source, others may be able to do so on your behalf (see WikiProject Resource Exchange).

Non-English sources


Citations to non-English reliable sources are allowed on the English Wikipedia. However, because this project is in English, English-language sources are preferred over non-English ones when they are available and of equal quality and relevance. As with sources in English, if a dispute arises involving a citation to a non-English source, editors may request a quotation of relevant portions of the original source be provided, either in text, in a footnote, or on the article talk page.[h] (See Template:Request quotation.)


If you quote a non-English reliable source (whether in the main text or in a footnote), a translation into English should accompany the quote. Translations published by reliable sources are preferred over translations by Wikipedians, but translations by Wikipedians are preferred over machine translations. When using a machine translation of source material, editors should be reasonably certain that the translation is accurate and the source is appropriate. Editors should not rely upon machine translations of non-English sources in contentious articles or biographies of living people. If needed, ask an editor who can translate it for you.

The original text is usually included with the translated text in articles when translated by Wikipedians, and the translating editor is usually not cited. When quoting any material, whether in English or in some other language, be careful not to

fair-use guideline

Other issues

Verifiability does not guarantee inclusion

While information must be verifiable for inclusion in an

presented instead in a different article
. The responsibility for achieving consensus for inclusion is on those seeking to include disputed content.

Tagging a sentence, section, or article

If you want to request an inline citation for an unsourced statement, you can tag a sentence with the {{

verification needed}}. Material that fails verification may be tagged with {{failed verification
}} or removed. It helps other editors to explain your rationale for using templates to tag material in the template, edit summary, or on the talk page.

Take special care with contentious material about living and recently deceased people. Unsourced or poorly sourced material that is contentious, especially text that is negative, derogatory, or potentially damaging, should be removed immediately rather than tagged or moved to the talk page.

Exceptional claims require exceptional sources

Any exceptional claim requires multiple high-quality sources.[4] Warnings (red flags) that should prompt extra caution include:

  • Surprising or apparently important claims not covered by multiple mainstream sources;
  • Challenged claims that are supported purely by
    or self-published sources or those with an apparent conflict of interest;
  • Reports of a statement by someone that seems out of character or against an interest they had previously defended;
  • Claims 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 and recently dead people. This is especially true when proponents say there is a conspiracy to silence them.

Verifiability and other principles

Copyright and plagiarism

Do not plagiarize or breach copyright when using sources. Summarize source material in your own words as much as possible; when quoting or closely paraphrasing a source, use an

in-text attribution
where appropriate.

Do not link to any source that violates the copyrights of others per contributors' rights and obligations. You can link to websites that display copyrighted works as long as the website has licensed the work or uses the work in a way compliant with fair use. Knowingly directing others to material that violates copyright may be considered contributory copyright infringement. If there is reason to think a source violates copyright, do not cite it. This is particularly relevant when linking to sites such as Scribd or YouTube, where due care should be taken to avoid linking to material violating copyright.


Even when information is cited to

inline citation
. Sources themselves do not need to maintain a neutral point of view. Indeed, many reliable sources are not neutral. Our job as editors is simply to summarize what reliable sources say.


If no reliable, independent sources can be found on a topic, Wikipedia should not have an article on it (i.e., the topic is not notable). However, notability is based on the existence of suitable sources, not on the state of sourcing in an article (


Original research


no original research
policy (NOR) is closely related to the Verifiability policy. Among its requirements are:

  1. All material in Wikipedia articles must be attributable to a reliable published source. This means a reliable published source must exist for it, whether or not it is cited in the article.
  2. Sources must support the material clearly and directly:
    drawing inferences from multiple sources to advance a novel position is prohibited by the NOR policy.[h]
  3. Base articles largely on reliable
    Misuse of primary sources
    section of the BLP policy.

See also


Information pages




  1. ^ This principle was previously expressed on this policy page as "the threshold for inclusion is verifiability, not truth". See the essay, Wikipedia:Verifiability, not truth.
  2. ^ a b c A source "directly supports" a given piece of material if the information is present explicitly in the source, so that using this source to support the material is not a violation of Wikipedia:No original research. The location of any citation—including whether one is present in the article at all—is unrelated to whether a source directly supports the material. For questions about where and how to place citations, see Wikipedia:Citing sources, Wikipedia:Manual of Style/Lead section § Citations, etc.
  3. unencyclopedic content; etc.). If necessary, all editors are then expected to help achieve consensus
    , and any problems with the text or sourcing should be fixed before the material is added back.
  4. tagging a section with {{unreferenced section}}, or the article with the applicable of either {{unreferenced}} or {{more citations needed}}. For a disputed category, you may use {{unreferenced category
    }}. For a disambiguation page, consider asking for a citation on the talk page.
  5. ^ When tagging or removing such material, please keep in mind such edits can easily be misunderstood. Some editors object to others making chronic, frequent, and large-scale deletions of unsourced information, especially if unaccompanied by other efforts to improve the material. Do not concentrate only on material of a particular point of view, as that may appear to be a contravention of Wikipedia:Neutral point of view. Also, check to see whether the material is sourced to a citation elsewhere on the page. For all these reasons, it is advisable to clearly communicate that you have a considered reason to believe the material in question cannot be verified.
  6. ^ This includes material such as documents in publicly accessible archives as well as inscriptions in plain sight, e.g. tombstones.
  7. ^ a b Note that any exceptional claim would require exceptional sources.
  8. ^ a b When there is a dispute as to whether a piece of text is fully supported by a given source, direct quotes and other relevant details from the source should be provided to other editors as a courtesy. Do not violate the source's copyright when doing so.


  1. ^ Wales, Jimmy. "Zero information is preferred to misleading or false information", WikiEN-l, May 16, 2006: "I can NOT emphasize this enough. There seems to be a terrible bias among some editors that some sort of random speculative 'I heard it somewhere' pseudo information is to be tagged with a 'needs a cite' tag. Wrong. It should be removed, aggressively, unless it can be sourced. This is true of all information, but it is particularly true of negative information about living persons."
  2. ^ Self-published material is characterized by the lack of independent reviewers (those without a conflict of interest) validating the reliability of the content. Further examples of self-published sources include press releases, the material contained within company websites, advertising campaigns, material published in media by the owner(s)/publisher(s) of the media group, self-released music albums, and electoral manifestos:
    • The University of California, Berkeley, library states: "Most pages found in general search engines for the web are self-published or published by businesses small and large with motives to get you to buy something or believe a point of view. Even within university and library web sites, there can be many pages that the institution does not try to oversee."
    • Princeton University offers this understanding in its publication, Academic Integrity at Princeton (2011): "Unlike most books and journal articles, which undergo strict editorial review before publication, much of the information on the Web is self-published. To be sure, there are many websites in which you can have confidence: mainstream newspapers, refereed electronic journals, and university, library, and government collections of data. But for vast amounts of Web-based information, no impartial reviewers have evaluated the accuracy or fairness of such material before it's made instantly available across the globe."
    • The Chicago Manual of Style, 16th Edition states, "Any site that does not have a specific publisher or sponsoring body should be treated as unpublished or self-published work."
  3. PMID 25272616
  4. OL 7067396M at para. 91) "A wise man ... proportions his belief to the evidence ... That no testimony is sufficient to establish a miracle, unless the testimony is of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavors to establish; and even in that case there is a mutual destruction of arguments, and the superior only gives us an assurance suitable to that degree of force, which remains, after deducting the inferior." In the 18th century, Pierre-Simon Laplace reformulated the idea as "The weight of evidence for an extraordinary claim must be proportioned to its strangeness." Marcello Truzzi recast it again, in 1978, as "An extraordinary claim requires extraordinary proof." Carl Sagan, finally, popularized the concept broadly as "Extraordinary claims require extraordinary evidence" in 1980 on Cosmos: A Personal Voyage
    ; this was the formulation originally used on Wikipedia.

Further reading

  • Wales, Jimmy. "Insist on sources", WikiEN-l, July 19, 2006: "I really want to encourage a much stronger culture which says: it is better to have no information, than to have information like this, with no sources."—referring to a rather unlikely statement about the founders of Google throwing pies at each other.