In research, the term tertiary source is a relative term. What is considered tertiary depends on what is considered primary and secondary. A tertiary source may thus be understood as a selection, distillation, summary or compilation of primary sources, secondary sources, or both. The distinction between primary source and secondary source is standard in historiography, while the distinction between these sources and tertiary sources is more peripheral, and is more relevant to the scholarly research work than to the published content itself.
In some contexts typical instances of tertiary sources are bibliographies, library catalogs, directories, reading lists and survey articles. Encyclopedias and textbooks are examples of written materials that typically embrace both secondary and tertiary sources, presenting on the one hand commentary and analysis, while on the other attempting to provide a synoptic overview of the material available on the topic.
The classification of a given source is subjective and contextual. For example, a modern encyclopedia might be generally considered a tertiary source, but an ancient encyclopedia is generally considered a primary source. The difference is because the modern encyclopedia is used as a source for information about a topic, and the ancient encyclopedia is understood as a source of information about the state of knowledge in the time period when the encyclopedia was written. However, even a modern encyclopedia is a primary source for some facts, such as the name of its publisher.
A different definition is used by the UNISIST model in which secondary sources are understood as bibliographies, while tertiary sources are understood as synthesis of primary literature.
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