What constitutes a primary source varies by discipline.
In the social sciences the terms primary literature and primary data are preferred over primary source. The former imply the results of original research or experimentation, usually gathered in the field and later reported in the peer-reviewed journal literature. Disciplinary examples are too numerous to list but include anthropologists who undertake dirt archaeology and ethnographic fieldwork; education researchers who gather data in the classroom; linguists who record communication patterns in a particular language community; and political scientists who conduct voter polls and surveys. In many social sciences disciplines a fine-grained distinction is made between the primary literature (i.e., the scholarly articles and presentations that report on original research) and the primary data (i.e., the "raw" data collected by researchers through interviews, observations, and survey instruments).
In the arts and humanities primary sources are original, creative works. These include photographs, paintings, prints, sculptures, ceramics, textiles and other studio works; original literary creations such as novels, plays, short stories, and poems; musical compositions and performances; and in architecture buildings, architectural complexes, and even whole cities. Art historians greatly prefer, when possible, to study original artworks yet often make do with re-productions. An art historian studying Olmec sculpture, for instance, might work with high-quality illustrations published in an exhibition catalog or found in library databases such as ARTstor. Preferred would be to study Olmec sculptures at the museum. Best of all would be the opportunity to study the sculptures in situ. The latter may be difficult or even impossible if the original archaeological context has been lost.
In history primary sources are "original records created at the time historical events occurred or well after events in the form of memoirs and oral histories. Primary sources may include letters, manuscripts, diaries, journals, newspapers, speeches, interviews, memoirs, documents produced by government agencies such as Congress or the Office of the President, photographs, audio recordings, moving pictures or video recordings, research data [social scientists' primary data, described above], and objects or artifacts such as works of art or ancient roads, buildings, tools, and weapons. These sources serve as the raw material to interpret the past..." (Primary Sources on the Web, a website of the History Section of RUSA, American Library Association). Primary sources are available in archives or as re-productions of originals published online or in books widely held by academic libraries.
What constitutes a secondary source is dependent on the discipline but the difference is largely semantic.
In the social sciences a secondary source is based on or written about the primary literature. A secondary source analyzes, editorializes, reports on, or summarizes the primary literature. While secondary sources in the social sciences are sometimes scholarly (e.g., a scholarly book that summarizes a body of primary literature or a review article published in an academic journal), more typically secondary sources in the social sciences appear in credible but popular publications. Examples include general-interest magazines such as Newsweek and Time, highbrow publications such as The Atlantic and New Yorker, newspapers such as The New York Times, and books and websites aimed at non-specialized audiences. The "conversation" social scientists have with one another occurs not in these popular publications but rather within the primary literature.
In the arts, humanities, and history a secondary source is also based on or written about primary sources. A secondary source in the humanities, however, is typically if not always scholarly (e.g., a book aimed at other scholars or an article published in a peer-reviewed journal). Historians, for instance, explore the significance of Thomas Jefferson's writings (i.e., the primary sources) in scholarly books and peer-reviewed journal articles written by and for other scholars.
Scholars in all disciplines work to address unresolved disciplinary problems and questions. At the same time, scholars strive to complicate what is already known. In all cases such "conversations" nearly always occur within the pages of scholarly books (i.e., monographs and edited volumes), and in peer-reviewed journal articles. Students will typically cite these sources in research papers.
You might encounter a tertiary source, more commonly called a reference work. Two layers removed from the primary evidence, tertiary sources are based on secondary sources. They contextualize or summarize secondary sources or list, index, or in some other way promote their efficient discovery. Reference works provide researchers with the information needed to conduct further research. Types of reference works include bibliographies, biographies, catalogs, chronologies, companions, dictionaries, directories, encyclopedias, handbooks, and indexes.
Scholars do more than present what is already known. Academics from undergraduates to senior scholars develop cogent arguments in support of their theses. The baseline information contained in encyclopedia articles cannot substitute for original scholarship. This is one reason why reference works are not generally cited in scholars' own bibliographies or reference lists. And yet each type or class of information—primary, secondary, and tertiary—fulfills a critical function in the research process.
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David C. Murray