Documentary films can be considered a secondary source or primary source, depending on how much the filmmaker modifies the original sources.
Although many primary sources remain in private hands, others are located in archives, libraries, museums, historical societies, and special collections. Some are affiliated with universities and colleges, while others are government entities.
In contexts such as historical writing, it is almost always advisable to use primary sources and that "if none are available, it is only with great caution that [the author] may proceed to make use of secondary sources." as a way to trace the history of historical ideas, they have been applied to many other fields.
For example, these ideas may be used to trace the history of scientific theories, literary elements and other information that is passed from one author to another.
For example, a memoir would be considered a primary source in research concerning its author or about his or her friends characterized within it, but the same memoir would be a secondary source if it were used to examine the culture in which its author lived.
"Primary" and "secondary" should be understood as relative terms, with sources categorized according to specific historical contexts and what is being studied.Primary sources are distinguished from secondary sources, which cite, comment on, or build upon primary sources.Generally, accounts written after the fact with the benefit (and possible distortions) of hindsight are secondary.Similar definitions can be used in library science, and other areas of scholarship, although different fields have somewhat different definitions.In journalism, a primary source can be a person with direct knowledge of a situation, or a document written by such a person.In the history of ideas or intellectual history, the main primary sources are books, essays and letters written by intellectuals; these intellectuals may include historians, whose books and essays are therefore considered primary sources for the intellectual historian, though they are secondary sources in their own topical fields.In religious history, the primary sources are religious texts and descriptions of religious ceremonies and rituals.Historians may also take archaeological artifacts and oral reports and interviews into consideration. In historiography, when the study of history is subject to historical scrutiny, a secondary source becomes a primary source.For a biography of a historian, that historian's publications would be primary sources.The National Archives and Records Administration also has digital collections in Digital Vaults.The Digital Public Library of America searches across the digitized primary source collections of many libraries, archives, and museums.