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PRIMARY SOURCES are publications that report the results of original research. They may be in the form of conference papers, monographic series, technical reports, theses and dissertations, or journal articles. Because they present information in its original form (that is, it has not been interpreted or condensed or otherwise “repackaged” by other writers), these are considered primary sources. The works present new thinking/discoveries/results and unite them with the existing knowledge base. Journal articles that report original research are one of the more common and important steps in the information sharing cycle. They often go through a process in which they are “peer reviewed” by other experts who evaluate the work and findings before publication.
SECONDARY SOURCES are those which are published about the primary literature, that generalize, analyze, interpret, evaluate or otherwise “add value” to the original information, OR which simplify the process of finding and evaluating the primary literature. Some examples of secondary sources are “review” articles and indexes or bibliographies, such as PubMed or the ScienceDirect.
TERTIARY SOURCES compile or digest information from primary or secondary sources that has become widely accepted. They aim to provide a broad overview of a topic, or data, already proven facts, and definitions, often presented in a convenient form. They provide no new information. These include “reference” types of works such as textbooks, encyclopedias, fact books, guides and handbooks, and computer databases such as The Handbook of the Microbiological Media and SciFinder.
GRAY LITERATURE are source materials not available through the usual systems of publication (e.g., books or periodicals) and distribution. Gray literature includes conference proceedings, dissertations, technical reports, and working papers. Locating this type of literature is a little more difficult, but there are finding tools such as Dissertations Abstracts and PapersFirst.