A tertiary source is an index and/or textual consolidation of primary and secondary sources. Some tertiary sources are not to be used for academic research, unless they can also be used as secondary sources, or to find other sources.
Overlap with secondary sources
Depending on the topic of research, a scholar may use a bibliography, dictionary, or encyclopedia as either a tertiary or a secondary source. This causes difficulty in defining many sources as either one type or the other.
In the United Nations International Scientific Information System (UNISIST) model, a secondary source is a bibliography, whereas a tertiary source is a synthesis of primary sources.
Types of tertiary sources
As tertiary sources, encyclopedias, textbooks, and compendia attempt to summarize and consolidate the source materials into an overview, but may also present subjective commentary and analysis (which are characteristics of secondary sources).
Survey or overview articles are usually tertiary, though review articles in peer-reviewed academic journals are secondary (not be confused with film, book, etc. reviews, which are primary-source opinions).
Some usually primary sources, such as user guides and manuals, are secondary or tertiary (depending on the nature of the material) when written by third parties.
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