In the English Wikipedia, verifiability means other people using the encyclopedia can check that the information comes from a reliable source. Wikipedia does not publish original research. Its content is determined by previously published information rather than the beliefs or experiences of editors. Even if you are sure something is true, it must be verifiable before you can add it. If reliable sources disagree, then maintain a neutral point of view and present what the various sources say, giving each side its due weight.
All material in Wikipedia mainspace, including everything in articles, lists and captions, must be verifiable. All quotations, and any material whose verifiability has been challenged or is likely to be challenged, must include an inline citation that directly supports the material. Any material that needs a source 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.
Attribute all quotations, and any material whose verifiability is challenged or likely to be challenged, to a reliable, published source using an inline citation. 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 a reliable source directly supporting it may be removed and should not be restored without an inline citation to a reliable source. Whether and how quickly material should be initially removed for not having an inline citation to a reliable source depends on the material and the overall state of the article. In some cases, editors may object if you remove material without giving them time to provide references; consider adding a citation needed tag as an interim step. When tagging or removing material for lacking an inline citation, please state your concern that it may not be possible to find a published reliable source and the material therefore may not be verifiable. If you think the material is verifiable, 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 or existing groups, and do not move it to the talk page. You should also be aware of how Wikipedia:Biographies of living persons applies to groups.
The word "source" when citing sources on Wikipedia has three related meanings:
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 our purposes is "made available to the public in some form". 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. The best sources have a professional structure in place 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. Be especially careful when sourcing content related to living people or medicine.
If available, academic and peer-reviewed publications are usually the most reliable sources in topics such as history, medicine, and science.
Editors may also use material from reliable non-academic sources, particularly if it appears in respected mainstream publications. Other reliable sources include:
Some newspapers, magazines, and other news organizations host online columns they call blogs. These may be acceptable sources if the writers are professionals, but use them with caution because blogs may not be subject to the news organization's normal fact-checking process. If a news organization publishes an opinion piece in a blog, attribute the statement to the writer, e.g. "Jane Smith wrote ..." Never use as sources the blog comments that are left by readers. For personal or group blogs that are not reliable sources, see § Self-published sources below.
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.
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 that are widely considered by other sources to be extremist or promotional, or that rely 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 also questionable, due to lack of effective peer-review.
Anyone can create a personal web page, self-publish a book, or claim to be an expert. That is why self-published material such as books, patents, newsletters, personal websites, open wikis, personal or group blogs (as distinguished from newsblogs, above), content farms, Internet forum postings, and social media postings are largely not acceptable as sources. Self-published expert sources may be considered reliable when produced by an established subject-matter expert, whose work in the relevant field has previously been published by reliable, independent publications. Exercise caution when using such sources: if the information in question is suitable for inclusion, someone else will probably have published it in independent reliable sources. Never use self-published sources as third-party sources about living people, even if the author is an expert, well-known professional researcher, or writer.
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 be published experts in the field, so long as:
Do not use articles from Wikipedia (whether this English Wikipedia or Wikipedias in other languages) as sources. Also, do not use websites that mirror Wikipedia content or publications that rely on material from Wikipedia as sources. Content from a Wikipedia article is not considered reliable unless it is backed up by citing reliable sources. Confirm that these sources support the content, then use them directly. (There is also a risk of circular reference/circular reporting when using a Wikipedia article or derivative work as a source.)
An exception is allowed when Wikipedia itself is being discussed in the article, which 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 primary source in this case, and may be used following the policy for primary sources. Any such use should avoid original research, undue emphasis on Wikipedia's role or views, and inappropriate self-reference. The article text should make it clear the material is sourced from Wikipedia so the reader is aware of the potential bias.
Some reliable sources may not be easily accessible. For example, an online source may require payment, and a print-only source may be available only in university libraries. Rare historical sources may even be available only in special museum collections and archives. Do not reject reliable sources just because they are difficult or costly to access. If you have trouble accessing a source, others may be able to do so on your behalf (see WikiProject Resource Exchange).
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 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. (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 always 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 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.
In articles, the original text is usually included with the translated text 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 violate copyright; see the fair-use guideline.
While information must be verifiable to be included in an article, all verifiable information need not be included in an article. Consensus may determine that certain information does not improve an article, and that it should be omitted or presented instead in a different article. The onus to achieve consensus for inclusion is on those seeking to include disputed content.
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.
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 inline citation, and 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 that violates copyright.
Even when information is cited to reliable sources, you must present it with a neutral point of view (NPOV). Articles should be based on thorough research of sources. All articles must adhere to NPOV, fairly representing all majority and significant-minority viewpoints 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. If there is disagreement between sources, use in-text attribution: "John Smith argues X, while Paul Jones maintains Y," followed by an 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 the reliable sources say.
The no original research policy (NOR) is closely related to the Verifiability policy. Among its requirements are: