In most cases you will use secondary sources. Secondary sources are works that interpret the primary data, such as a book about eating disorders, a journal article about the role of tobacco in the colonial economy, or a critical review of a play. However, sometimes your assignment may require you to use primary sources such as diaries, interviews, letters, raw data, or original research.A primary source is an account by an eyewitness or the first recorder of an event in written or other form, including microform and electronic reproduction.