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Original materials that provide direct evidence or first-hand testimony concerning a topic or event.
Primary sources can be contemporary sources created at the time when the event occurred (e.g., letters and newspaper articles) or later (such as, memoirs and oral history interviews).
Primary sources may be published or unpublished. Unpublished sources are unique materials (e.g., family papers) often referred to as archives and manuscripts.
What constitutes a primary source varies by discipline. How the researcher uses the source generally determines whether it is a primary source or not.
2) Secondary Sources
Works that interpret, analyze, and discuss the evidence provided by primary sources (e.g., scholarly books and articles).
Secondary sources are generally a second-hand account or observation at least one step removed from the event.
Secondary sources, however, can be considered to be primary sources depending on the context of their use. For example, Ken Burns' documentary of the Civil War is a secondary source for Civil War researchers, but a primary source for those studying documentary filmmaking.
3) Tertiary Sources
Books or articles that synthesize or distill primary and secondary sources, often in a convenient, easy-to-read form (e.g., dictionaries, encyclopedias, indexes, and textbooks).
Example . . .
The Historian researching World War I might utilize:
Newspaper articles, weekly/monthly news magazines, diaries, correspondence, and diplomatic records from 1914 to 1919.
Articles in scholarly journals analyzing the war, possibly footnoting primary documents; books analyzing the war.
The Literary Critic researching literature written during World War I might utilize:
Novels, poems, plays, diaries, and correspondence of the time period.
Published articles in scholarly journals providing analysis and criticism of the literature; books analyzing the literature; formal biographies of writers from the era.
The Psychologist researching trench warfare and post-traumatic stress disorder in World War I veterans might utilize:
Original research reports on the topic or research notes taken by a clinical psychologist working with World War I veterans.
Articles in scholarly publications synthesizing results of original research; books analyzing results of original research.
The Scientist researching long-term medical effects of chemical warfare on exposed veterans might utilize:
Published articles in scholarly journals reporting on a medical research study and its methodology.
Published articles in scholarly journals analyzing results of an original research study; books doing the same.