It is important that sources are chosen and used properly in Wikipedia. References must be reliable sources, used in accordance with the three core content policies. For stylistic and formatting issues, please refer to Wikipedia:Manual of Style. For the suitability of certain types of content, see Wikipedia:What Wikipedia is not. For guidance on the encyclopedic suitability of subjects or articles as a whole, refer to Wikipedia:Notability. For articles about living people, see Wikipedia:Biographies of living persons.
Research that consists of collecting and organizing material from existing sources within the provisions of the content policies is, of course, encouraged: this is "source-based research," and it is fundamental to writing an encyclopedia. However, consideration should be given to the kind of source used, to ensure that articles properly represent generally accepted knowledge about a given topic. Sources can be divided into two broad categories for these purposes, based upon their quality and reliability: reliable third-party sources and auxiliary sources.
"Verifiability" requires that the facts and claims we present are verifiable by other editors. Any material that is challenged or likely to be challenged must be supported by a reliable source. "Original research" is a claim, including interpretation or analysis of facts and references, for which no reliable source can be found. Producing a reliable published source that advances the same claim taken in context is the only way to disprove original research. Citing reliable published sources is essential in our goal of building a quality reference work.
Reliable third-party sources
Reliable third-party sources should be the principal reference material for Wikipedia articles, whenever possible. These sources are up-to-date, written purposefully to inform about the subject they are being cited for, and released by a publisher with a reputation for fact-checking and accuracy. Third-party publications are preferable because they generally provide analysis, offer a more independent view and provide a broader context for the subject. Care should be taken to avoid undue weight and ensure the information cited is used in context.
Using reliable third-party sources
While articles should principally rely on reputable third-party sources, care should still be taken that the claims reported in the article are verifiable in the references cited. Claims should not rely on unclear, incongruent or passing comments, even if the source is generally reliable. Claims left open to interpretation should be precisely cited or avoided. Drawing conclusions not stated in the reference, or extrapolating a position from the claims in a source, is original research regardless of the type of source. Claims based on statements and sections from reliable sources directly dealing with the central topic of the work are preferred. It is good to report information from sections that present an extended argument with a conclusion strongly consistent with the argument. It is important that reliable references are cited in context and on topic.
Auxiliary sources should only be used with care, or in context as used in reliable third-party sources. References from questionable, historical and "raw" sources are examples of auxiliary sources. References from the subject, or those close to the subject, are also examples of auxiliary sources. A third-party source from one article may be treated as an auxiliary source in another, because the focus has changed. Generally, any source that does not qualify as a reliable third-party source is grouped in this category. The use of these raw, first-hand, or out-of-date sources lends itself to inaccurate reporting, undue weight, and original research. Thus, they should only be used with caution and extreme care.
Using auxiliary sources
Articles should usually rely on reliable third-party sources, but there are some exceptions and occasions when auxiliary sources may prove useful. Article claims that rely on an auxiliary source should (1) only report what the source states, the accuracy of which is easily verifiable by any reasonable, educated person without specialist knowledge, and (2) make no analytic, synthetic, interpretive, explanatory, or evaluative claims. Contributors drawing on auxiliary sources should be careful to comply with both conditions. Of course, auxiliary sources may be used freely as they are used in reliable publications.
Some of these sources may be particularly reliable and helpful in presenting a complete encyclopedic article, such as census data. Some auxiliary references may also be useful for providing supporting facts, figures or limited quotations to accompany claims and analysis from reliable third-party publications. However, serious care should be taken to avoid presenting a claim or interpretation, explicit or implied, differing from the reliable references cited. Self-published and dubious sources are permitted in articles about themselves, when used with care. Editors should use discretion and consensus to identify such circumstances.
- Various professional fields treat the types of sources in differing fashions, but such distinctions are not our concern here. The main focus here is distinguishing between which references articles should principally rely on, and which references have a tendency to be misused.
- Review articles and references providing a broad treatment, such as university-level textbooks, are particularly useful in summarizing large bodies of literature and research. Since Wikipedia summarizes broad knowledge on topics, such sources are valuable references.
- This is not to say they are more objective. These sources are simply independent from the subject.
- Examples of such references would be recent post-secondary textbooks, or a contemporary work released by a reputable publisher. Peer-reviewed articles in reputable journals are also considered reliable third-party publications.
- Examples include notable conspiracy theory websites, Bede's Historia ecclesiastica gentis Anglorum and transcripts of the Nixon tapes.
- For example, a book by a notable historian is a reliable third-party source for the events it covers, but it would be considered an auxiliary source in the article about the historian.
- Also, there is a broad consensus for widespread use of some auxiliary sources, such as using census data in articles on populated places and citing interviews as direct sources of a subject's self-identification, opinions and statements.
- In that instance, an editor would be relying on a reliable third-party reference to present, analyze and/or interpret an auxiliary source. The third-party publication helps avoid original research, recognize due weight and identify context.