Primary sources are the "materials on a topic upon which subsequent interpretations or studies are based, anything from firsthand documents such as poems, diaries, court records, and interviews to research results generated by experiments, surveys, ethnographies, and so on."*
Primary sources are records of events as they are first described, usually by witnesses or people who were involved in the event. Many primary sources were created at the time of the event but can also include memoirs, oral interviews, or accounts that were recorded later.
Visual materials, such as photos, original artwork, posters, and films are important primary sources, not only for the factual information they contain, but also for the insight they may provide into how people view their world. Primary sources may also include sets of data, such as census statistics, which have been tabulated but not interpreted. However, in the sciences or social sciences, primary sources report the results of an experiment.
It can sometimes be difficult to determine whether a particular source is primary or secondary, because the same source can be a primary source for one topic and a secondary source for another topic. David McCullough’s biography, John Adams, could be a secondary source for a paper about John Adams but a primary source for a paper about how various historians have interpreted the life of John Adams.
*From Hairston, Maxine and John J. Ruszkiewicz. The Scott, Foresman Handbook for Writers. 4th ed. New York: HarperCollins College Publishers, 1996, pg. 547.
Secondary sources offer an analysis or a restatement of primary sources. They often attempt to describe or explain primary sources. Some secondary sources not only analyze primary sources, but also use them to argue a contention or persuade the reader to hold a certain opinion. Examples of secondary sources include dictionaries, encyclopedias, textbooks, and books and articles that interpret, analyze, or review research works.