[[Category:Wikipedia {{{1}}}s|Reliable sources]]

There is general agreement that Wikipedia articles should use reliable sources. This page is an attempt to provide guidance about how to identify reliable published sources. The two policy pages that discuss the need to use sources are Wikipedia:No original research and Wikipedia:Verifiability.

If you can provide useful information to Wikipedia, please do so, but bear in mind that edits for which no credible references are provided may be deleted by any editor.

What follows is a description of Wikipedia's best practises. Many articles may fall short of this standard until one or more editors devote time and effort to fact-checking and reference-running. See Efforts to identify reliable sources at the bottom of this page for collaborative projects to do just that. In the meantime, readers can still benefit greatly from your contributions, bearing in mind that unsourced edits, or edits relying on inappropriate sources, may be challenged at any time.

There are many ways in which factual errors can be introduced into reports. Different types of claims may require different methods for determining their accuracy. Keep in mind that some articles are about characterizing the various factions in a dispute. This means that you will be looking for reliable published reports of people's opinions.

Apart from credibility, another aspect of reliability is treated in this guideline: the availability of cited sources.

Please note the following terms:

• A fact is an actual state of affairs, which can be an historical event, or a social or natural phenomenon. To say of a sentence or proposition that it is true is to say that it refers to a fact.

• An opinion is a view that someone holds, the content of which may or may not be verifiable. However, that a certain person or group of people holds a certain opinion is a fact, and it may be included in Wikipedia if it can be verified.

• A primary source provides direct evidence for a certain state of affairs. This may mean that the source observes a state of affairs directly, or that they observe indirect evidence of it. In other words, a primary source is a source very close to the original state of affairs you are writing about. An example of primary-source material would be a photograph of a car accident taken by an eye witness, or a report from that eye witness. A trial transcript is also primary-source material. Wikipedia articles may rely on primary sources so long as what they say has been published by a credible publication. For example, a trial transcript has been published by the court. We may not use primary sources whose information has not been made available by a credible publication. See Wikipedia:No original research.

• A secondary source summarizes one or more primary or secondary sources. A tertiary source usually summarizes secondary sources.

When reporting facts, Wikipedia articles should cite sources. Tertiary sources like reputable encyclopedias, such as the Encyclopædia Britannica, as well as reputable specialized encyclopedias such as the New Grove and the Dictionary of National Biography, contain a wealth of reliable information. Older editions such as the 1911 Encyclopædia Britannica often have fuller articles than current editions on some subjects, though there is always the danger that the information is outdated.

If suspect sources have references, follow them. If there are no references, or if the references provided are insufficient, you may need to do additional research, or reconsider the reliability of the report.

When reporting that an opinion is held by a particular individual or group, the best citation will be to a direct quote, citing the source of the quote in full after the sentence (see Harvard referencing) or using a footnote. If there is text, audio, or video available of someone expressing the opinion directly, it is preferable to include or transcribe an excerpt, which is allowed under fair use.

Wikipedians often report as facts things they remember hearing about or reading somewhere, but they don't remember where, and they don't have any other corroborating information. It's important to seek credible sources to verify these types of reports, and if they cannot be verified, any editor may delete them.

It's always appropriate to ask other editors, "How do you know that?", or "Can you cite your source?" If they didn't have a particular source in mind when they wrote the material originally, someone will have to find a credible source. The burden of evidence lies with the editor who has made the edit in question.

For advice about dealing with unattributed material when you find it, see Wikipedia:Verifiability and Wikipedia:No original research, which are policy, and Wikipedia:Cite sources, which is a style guide.

• Do they have an agenda or conflict of interest, strong views, or other bias which may color their report? Remember that conflicts of interest are not always explicitly exposed and bias is not always self-evident. However, that a source has strong views is not necessarily a reason not to use it, although editors should avoid using political groups with widely acknowledged extremist views, like Stormfront.org or the Socialist Workers' Party. Groups like these may be used as primary sources only i.e. as sources about themselves, and even then with caution and sparingly.
• Were they actually there? Be careful to distinguish between descriptions of events by eyewitnesses and by commentators. The former are primary sources; the latter secondary. Both can be reliable.
• Find out what other people say about your sources.
• Have they reported other facts reliably, including on different subjects? Cross-check with what you already know.
• Are the publications available for other editors to check? We provide sources for our readers, so they must be accessible in principle, although not necessarily online.

See Wikipedia:No original research and Wikipedia:Verifiability for more information.

Psychological experiments have shown that memory and perception are not as reliable as we would like them to be. In one experiment, subjects were shown playing cards with some anomalies. Subjects could usually identify normal cards correctly if even they were displayed for a very short amount of time. But when briefly flashed a black four of hearts, for example, most subjects would, without apparent hesitation or puzzlement, incorrectly identify it as either the four of hearts or the four of spades. Subject only became aware of the anomalous cards with longer exposures. Subjects who were aware that these strange cards were mixed in with normal cards were also much better at identifying them. ^[1]

Recent scientific experiments have begun to explain how the brain can remember imagined events as if they were real. ^[2][3] Police, judges, and trial lawyers are familiar with the phenomenon that several different people witnessing the same event remember it differently, sometimes including crucial details. A common classroom exercise is to stage a sudden interruption of the class, then ask each student to write an account of what he or she saw. Most people also know that the older a memory is, the less reliable it may be. Recent studies have shown that this may be because memories are overwritten each time we access them. ^[4]

Because conscious and unconscious biases are not always self-evident, you shouldn't necessarily be satisfied with a single source. Find another one and cross-check. If multiple independent sources agree and they have either no strong reason to be biased, or their biases are at cross purposes, then you may have a reliable account.

However, bear in mind that it is not the job of Wikipedians to do original research. We report what reliable publications publish. We do not investigate whether they are right or wrong. See Wikipedia:No original research.

• Have they used multiple independent primary sources?
• Do they have an agenda or conflict of interest, strong views, or other bias which may color their report? Remember that conflicts of interest are not always explicitly exposed and bias is not always self-evident. However, that a source has strong views is not necessarily a reason not to use it, although editors should avoid using political groups with widely acknowledged extremist views, like Stormfront.org or the Socialist Workers' Party. Groups like these may be used as primary sources only i.e. as sources about themselves, and even then with caution and sparingly. Extremist groups should not be used as secondary sources.
• Find out what other people say about your sources.
• Have they reported other facts reliably, including on different subjects? Cross-check with what you already know.
• Are they available to other editors to check? If not, inclusion is probably not appropriate, though they need not be online. Availability through a library is sufficient.

Even given the same primary sources, different analysts may come to different conclusions about the facts being reported. In practice, many secondary sources find and use different primary sources in the course of their research. Conscious biases, unconscious biases, and errors are not always self-evident. The best way to expose them is to cross-check with an independent source.

Independent secondary sources:

• There is separate editorial oversight. This means that they have different employers, or different editors (but not necessarily different publishers).
• Have not collaborated their efforts.
• May have taken their own look at the available primary sources and used their own judgment in evaluating them.

Certain "red flags" should prompt editors to examine closely and skeptically the sources for a given claim.

• Reports of a statement by someone that seems out of character, embarrassing, controversial, or against an interest they had previously defended.
• Surprising or important facts which are not widely known.
• Surprising or important recent events which have not been reported by reputable news media.
• Claims which are not supported, or which are contradicted, by the prevailing view in the scientific community.
• Claims which strongly support one or another party in an ongoing dispute (see e.g. Wikipedia:List of controversial issues)
• Note that none of the above would necessarily exclude a source from being used.

Another aspect in choosing sources, is that it is preferable that sources are reachable by the users of the wikipedia encyclopedia: for printed resources that's not so difficult to understand: if there is a book printed on 10.000 exemplaries, and another book printed on 100.000 examplaries, widely spread in libraries, and if both books would be as usable to support a statement in a wikipedia article, of course the preference goes to the book that is most widely spread. Similarly, if two books make the same statement, and one is in English, while the other only available in foreign languages, English wikipedia prefers to make the reference to the English book.

For web sources the availability aspect also translates in the stability of the websites to which one makes references, which is the topic for the next subsection:

When a link that was present just for an external links section "goes dead", it is no big deal, and we can get rid of it. Reference links are another matter. In general, they are still worth keeping as part of the referencing apparatus of the article; often, a live substitute link can be found. Here are some pointers. In most cases, these approaches will preserve an acceptable citation.

• A very large proportion of pages can be recovered from the Internet Archive. Just go to http://www.archive.org/ and search for the old link by URL. Make sure that your new citation mentions the date the page was archived by the Internet Archive.
• If this was a non-blind citation of web-only material, it may be worth the effort to poke around the target site for an equivalent page at a new location, an indication that the whole site has moved, etc.
• If the link was merely a "convenience link" to an online copy of material that originally appeared in print, and an appropriate substitute cannot be found, it is OK to drop the link but keep the citation.
• If you cannot find the page on the Internet Archive, remember that you can often find recently deleted pages in Google's cache. They won't be there long, and it is no use linking to them, but this may let you find the content, which can be useful in finding an equivalent page elsewhere on the Internet and linking to that.

Wikipedia does not currently have a policy for what to do when none of those strategies succeed, but here are some suggestions.

• Be careful not to remove a link prematurely. If you reach the site, and it says the particular page is gone, that is pretty definitive, but one failure to access a site does not mean it is permanently dead. In the latter case, note (in an HTML comment) the date on which the link seemed dead.
• You can place the link in an HTML comment, marked as dead. Date the comment. The Internet Archive deliberately lags by six months, so there is a fair chance that at any time in the next six months we might, again, be able to get an equivalent link.
• If the link has been dead more than six months, keep it (commented out) as the record of a reference used.

Historical events are difficult to verify. We must rely on people's memories, recorded accounts, and physical evidence to reconstruct it. Evidence and factual accounts contemporary with the events are valuable because there are fewer steps separating them from reality. On the other hand, their authors are more likely to have been involved in the events, and therefore to have a particular point of view. Indeed, the evaluation and judicious use of primary sources is a major part of the craft of history.

Summaries and overviews of history require interpretation and analysis, finding patterns and attributing causes. Sometimes later historical analyses of this kind are more reliable, because the passage of time allows more scholarly debate, more reflection, and decreases the likelihood that the historian was personally involved in or attached to the events that he or she is analyzing.

Scientific journals are the best place to find primary source articles about scientific experiments, including medical studies. The best scientific journals are peer-reviewed, which means that independent experts in the field are asked to (usually anonymously) review articles before they are published. This usually results in corrections and improvement, sometimes substantial. Many articles are excluded from peer-reviewed journals because they report questionable (or perhaps merely unimportant, in the opinion of the editors) results.

• The fact that a statement is published in a peer-viewed journal does not make it true.

Even a well-designed experiment or study can produce flawed results. (For example, see the Retracted article on neurotoxicity of ecstasy published in the very prestigious journal Science.) The second part of peer review occurs after publication. Colleagues in the field will read the journal article and discuss it in various forums, including other journals in the same field and often later articles in the same journal. They may find self-evident flaws in the procedure used just by reading the article and applying their experience, or it may take a long process of trying to reproduce the results by similar or completely different means for the scientific community to determine that the original results were corrupted by some undetermined methodological problem, or to rigorously confirm the original result. The most reliable source for scientific information is the prevailing "scientific consensus".

Determining the scientific consensus could be done with a survey of experts in the field, but it can also be accomplished by following the state of discussions in respected journals.

There is sometimes no one prevailing view because the available evidence does not yet point to a single answer. Because Wikipedia not only aims to be accurate, but also useful, it generally tries to explain the theories and empirical justification for each school of thought, with reference to published sources. Editors should not, however, create arguments themselves in favor of, or against, any particular theory or position. See Wikipedia:No original research, which is policy.

• Just because something is not an accepted scientific fact, as determined by the prevailing scientific consensus, does not mean that it should not be reported and referenced in Wikipedia.

However, although minority views are welcome in Wikipedia, the views of tiny minorities need not be reported. See Wikipedia:Neutral Point of View. Minority views should be reported as that, and should not be given the same amount of space in an article as the majority view.

Simply make readers aware of any uncertainty or controversy. A well-referenced article will point to specific journal articles or specific theories proposed by specific researchers, rather than: "Some say that ... (vague, unattributed theory), but others believe ... (vague, unattributed theory)."

A good way to determine which journals are held in high esteem without polling a bunch of scientists is to look at Impact Factor ratings, which track how many times a given journal is cited by articles in other publications.

Keep in mind that even a reputable journal may occasionally post a retraction of an experimental result. Articles may be selected on the grounds that they are interesting or highly promising, not merely because they seem reliable.

There are certain techniques that scientists use to prevent results from being contaminated by certain kinds of common errors, and to help others replicate results.

• Experimental control
  • Placebo controls
  • Ensuring demographic information aligns with the general population to check that the sample is sufficiently random
  • Double-blind medical studies
• Present a high degree of detail about the design and implementation of the experiment; don't just present the results.
• Make raw data available; don't just present conclusions.

Would you trust a plumber to fill your cavities? Likewise, you should probably not trust someone who has a Ph.D. in plant biology to tell you about quantum mechanics. Just as actors in TV commercials don white lab coats to make viewers think they are serious scientists, people with degrees in one field are not necessarily experts in any other. Watch out for false claims of authority.

Try to use sources who have degrees in the field they are discussing. The more reputable ones are affiliated with academic institutions. The most reputable have written textbooks in their field for the undergraduate level or higher: these authors can be expected to have a broad, authoritative grasp of their subject.

Statistical information is easily and often misinterpreted by the public, by journalists, and by scientists. It should be checked and explained with the utmost care, with reference to published sources.

See Misuse of statistics, Opinion poll, and Statistical survey for common errors and abuses.

Evaluate the reliability of online sources just as you would print or other more traditional sources. Neither online nor print sources deserve an automatic assumption of reliability by virtue of the medium they are printed in. All reports must be evaluated according to the processes and people that created them.

Publications with teams of fact-checkers, reporters, editors, lawyers, and managers — like the New York Times or The Times of London — are likely to be reliable, and are regarded as reputable sources for the purposes of Wikipedia.

At the other end of the reliability scale lie personal websites, weblogs (blogs), bulletin boards, and Usenet posts, which are not acceptable as sources.

Posts to bulletin boards and Usenet, or messages left on blogs, are never acceptable as primary or secondary sources.

This is because we have no way of knowing who has written or posted them.

A personal website or blog may be used only as a primary source i.e. when we are writing about the subject or owner of the website. But even then we should proceed with great caution and should avoid relying on information from the website as a sole source. This is particularly true when the subject is controversial, or has no professional or academic standing.

Personal websites and blogs may never be used as secondary sources.

That is, they may never be used as sources of information about a person or topic other than the owner of the website.

The reason personal websites are not used as secondary sources — and as primary sources only with great caution and not as a sole source if the subject is controversial — is that they are usually created by unknown individuals who have no one checking their work. They may be uninformed, misled, pushing an agenda, sloppy, relying on rumor and suspicion, or insane; or they may be intelligent, careful people sharing their knowledge with the world. It is impossible to know which is the case. Visiting a stranger's personal website is often the online equivalent of reading an unattributed flyer on a lamp post, and should be treated accordingly.

Partisan political and religious sources should be treated with caution, although political bias is not in itself a reason not to use a source. Widely acknowledged extremist political or religious websites — for example, Stormfront.org, Hamas, or the Socialist Workers' Party — should never be used as sources for Wikipedia, except as primary sources i.e. in articles discussing the opinions of that organization or the opinions of a larger like-minded group, but even then should be used with great caution, and should not be relied upon as a sole source.

Full-text online sources are preferable to offline sources if they are of similar quality and reliability, because they are easily accessed by other editors who want to check references, and by readers who simply want more information.

If you find a print source that is out of copyright or that is available on compatible licensing terms, add it to Wikisource and link to it there (in addition to the normal scholarly citation). Many significant out-of-copyright books have already been put online by other projects.

Until more authors publish online, and more material is uploaded, some of the most reliable and informative sources are still in print. If you can't find a good source on Google, go to your local library or bookstore. They exist for a reason. You'll be amazed what you can learn there.

• Wikipedia:WikiProject Fact and Reference Check
• Wikipedia:Forum for Encyclopedic Standards
• Wikipedia:Approval mechanism

• Wikipedia:Neutral Point of View, policy
• Wikipedia:No original research, policy
• Wikipedia:Verifiability, policy
• Wikipedia:Cite sources, style guide
• Wikipedia:Check your facts, style guide
• Wikipedia:Common knowledge, guideline

1. ^ The playing card experiment is described by Thomas Kuhn in The Structure of Scientific Revolutions (1962), p. 62-64. He cites the following article: J.S. Bruner and Leo Postman, "On the Perception of Incongruity: A Paradigm," Journal of Personality, XVIII (1949), 206-23. Following the advice of this page, the original source should be checked to see if the summary of Kuhn's summary is accurate.
2. ^ False memories based on imagined events: "Biological Basis for False Memories Revealed" by Michelle Trudeau. All Things Considered 23 Oct 2004. [5]
3. ^ "Making False Memories." Talk of the Nation Science Friday. 4 Feb 2005. [6]
4. ^ On overwriting memories each time we access them:

(Currently locating sources.)