Wikipedia:Classification of sources
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According to Wikipedia's verifiability policy, any material that is challenged or likely to be challenged must be accompanied by a reliable source. Research that consists of collecting and organizing material from existing sources is encouraged: this is "source-based research," and it is fundamental to writing an encyclopedia. Problems often arise, however, when editors use sources in ways that constitute original xsscx research, or which violate Wikipedia's policy of neutrality.
This guideline sets forth rules and factors that an editor should keep in mind while using various types of sources for verifying the statements in an article. Sources can be classified in various ways. The most useful classification is by their primary or secondary nature. Another classification is in terms of first-party or second-party.
Primary and secondary sources
Although there are several ways to classify sources, one of the most useful is by their primary or secondary nature, a distinction deriving from the field of Historiography. Although there is some variability in the use of these terms outside of Wikipedia, we focus on one particular aspect of the distinction useful for editors:
A primary source is a source cited for some new idea, creative thought, or data originating in that source, and not derived from another author or another source. Primary sources usually have some immediate connection or contact with the source of the new idea, thought, or data. For example, the primary source of some experimental data might be written by the scientist who performed the experiments. The primary source of a quotation might be written by someone who was present when the thing was said. The primary source of a historical theory is usually written by the historian who first conceived that theory. The primary source of information about a fictional universe is usually written by the author of that fictional universe.
A secondary source is any source cited for its second-hand information from a different work. Secondary sources are not the originators of new ideas, creative thoughts, or data; they merely act as a conduit for such information. For example, if an author compiles research data from several scientists into a table for comparison, she is a secondary source with respect to that data. If an author paraphrases a quotation in another source, she is a secondary source with respect to that quotation. If an author in a historiography summarizes a historical theory from the 1800s, she is a secondary source as to that historical theory. An encyclopedia about a fictional universe is a secondary source as to the works of fiction defining that fictional universe.
Some secondary sources, such as textbooks and treatises, are further described as tertiary sources. However, the tertiary source concept is not as significant and clear-cut as the others, and the category has less relevance to Wikipedia, except for the fact that Wikipedia is itself a tertiary source.
Primary and secondary are relative terms
Any source can be either a primary or secondary source, depending on how it is used. For example, if an author compiles research data from several scientists into a table for comparison, she is a secondary source with respect to that data. However, she might also make original conclusions about that data. The source would be primary with respect to those new conclusions. If an author paraphrases a quotation in another source, she is a secondary source with respect to that quotation. However, if she draws novel implications about that quotation, or synthesizes that quotation with other quotations, the work would be a primary source with respect to those new conclusions. If an author in a historiography summarizes a historical theory from the 1800s, she is a secondary source as to that historical theory. However, if she provides novel insights linking that historical theory with 1800s culture, the work is a primary source as to those original conclusions. An encyclopedia about a fictional universe is a secondary source as to the works of fiction defining that fictional universe. However, if the encyclopedia "fills in gaps" or makes novel generalizations, the encyclopedia is a primary source as to the author's unique contribution to the field.
Guidelines for primary and secondary sources
If given in their proper context, primary sources can be the most neutral and informative way to present information in a Wikipedia article. Often, however, the import or significance of primary sources is not obvious or is controversial, in which case they should be supported by secondary sources.
Non-controversial and respected secondary sources can be even more neutral and informative than primary sources. Sometimes, however, secondary sources act as filters and add "spin" to primary sources. Therefore, polemical or controversial secondary sources should be balanced with other secondary sources, and typically by reference to the unvarnished primary sources, so that the reader can have a basis to determine which secondary source provides the most credible "spin" on the primary sources.
When available, well-respected tertiary sources, such as textbooks and legal treatises, can be the most neutral secondary sources for use in Wikipedia articles. Frequently, however, the process by which the author collected the information is unclear and not well documented, and sometimes, the author is unknown. In such cases, tertiary articles should be supported by primary and other secondary sources.
First-party and third-party sources
Another way to categorize sources is by whether the cited information is written by the authoritative creator of that information (the "first party"), or by someone else (a "third party"). A source is considered third-party if the author and/or sponsoring/publishing organisation is not involved in the subject of the source. Thus, an autobiography is never third-party, as the author is the subject, and an article published by Microsoft on the reliability of Windows XP is not third-party, as the company is describing its own product. On the other hand, a technical review of Windows by someone not involved in operating system development or marketing is likely to be third-party, as is a military history from someone not involved in the conflict in question.
Third-party sources are generally preferred as the author has no obvious incentive to distort the truth or "spin" the facts a certain way. They are thus considered advantageous in ensuring a neutral point of view. However, it should be remembered that a source is not necessarily entirely neutral just because it is third-party, and where a range of views and perspectives exist, they should all be given reasonable coverage. In such situations, care should be taken to avoid giving undue weight to a particular point of view.