Primary, secondary and tertiary sources

Context determines whether a source is primary, secondary or tertiary. Sources that are normally considered to fit into one category may sometimes be used as another. For example, encyclopedias are normally used as tertiary resources, but a study of how encyclopedias have evolved through time would probably use them as primary sources. Each discipline has its own set of standards for what counts as a primary source; when in doubt, ask your professor.
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What they are and why it matters
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Primary sources
Primary sources allow researchers to get as close as possible to original ideas, events and empirical studies as possible. Such sources may include expositions of creative ideas, first hand or contemporary accounts of events, publication of the results of empirical observations or studies, and other items that may form the basis of further research. Examples include:
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Secondary sources
Secondary sources analyze, review or restate information in primary resources or other secondary resources. Even sources presenting facts or descriptions about events are secondary unless they are based on direct participation or observation. Moreover, secondary sources often rely on other secondary sources and standard disciplinary methods to reach results, and they provide the principle sources of analysis about primary sources. Examples include:
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Tertiary sources
Tertiary resources provide overviews of topics by synthesizing information gathered from other resources. Tertiary resources often provide data in a convenient form or provide information with context by which to interpret it. Examples include:
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