A library reference book serves a different purpose from a newspaper editorial, a magazine ad, or a Web site that promotes a service. To understand the purpose or intention of a source, ask critical questions: Is its purpose to explain or inform? To report new research? To persuade? To add a viewpoint? To sell a product? Does the source acknowledge its purpose in its preface, mission statement, or “About Us” or FAQ (Frequently Asked Questions) page?

Bias. A bias is a preference for a particular side of an issue. Because most authors and publishers have opinions on their topics, there’s little point in asking whether they are biased. Instead, ask how that viewpoint affects the presentation of information and opinion. What are the author’s or sponsor’s allegiances? Does the source treat one side of an issue more favorably than another? Is that bias hidden or stated? A strong bias does not invalidate a source. However, if you spot such bias early, you can look for other viewpoints to avoid lopsided analyses.

Primary or Secondary Information. A primary source is a firsthand account written by an eyewitness or a participant. It contains raw data and immediate impressions. A secondary source is an analysis of information in one or more primary sources. Primary sources for investigating the Korean War might include diaries or letters written by military personnel, accounts of civilian witnesses, articles by journalists on the scene, and official military reports. If a historian used those accounts as evidence in a study of military strategy or if a peace activist used them in a book on consequences of warfare for civilians, the resulting works would be secondary sources.

Most research papers benefit from both primary and secondary sources. If you repeatedly cite a fact or an authority quoted in someone else’s analysis, try to go to the primary source. After all, a bombing raid that spared 70 percent of a village also leveled 30 percent of it. The original research (published as a primary source) can help you learn where facts end and interpretation begins.