Tertiary source
The distinction between sources can be relative to a specific field of study. In some academic disciplines the distinction between a secondary and tertiary source can sometimes be confusing. Almanacs, fact books and handbooks can at times be considered tertiary while at other times secondary sources. Some manuals and textbooks would be considered tertiary sources while others would be secondary sources. Many of these sources have to be evaluated or vetted to determine which they are. Another view taken by the Purdue University guide (Writing a research paper, 2010) refers to the "distance" a source is from the original research. A primary source is the research; the raw data. A secondary source reports on primary sources and includes interpretation (or explanations), analysis and commentary. A tertiary source is the furthest from the original research. In most cases their purpose is to provide a general introduction to a topic. In many scholarly fields, tertiary sources are not used for research.