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The following definitions refer to published, available information. The information is put in a (reasonably) permanent form.
Primary Sources are the original resources that first report research or ideas. In research, these are often research articles in scholarly journals. However, they may include newspapers, research reports, trade journals, conference proceedings, dissertations, Web sites, novels, poems, plays, speeches, interviews, letters, case studies, test data, findings from surveys, archaeological drawings, experiments, films, drawings, designs, paintings, music, sculptures, etc. IF it is the original source of information.
Secondary Sources are resources that analyze, describe, and synthesize the primary or original source. These include review articles, newspaper articles, reference books such as encyclopedias and dictionaries, and textbooks.
This definition is a little vague. Tertiary sources are resources written about the secondary literature. In other words, literature which explains, teaches you how to use, and leads you through the vast array of primary and secondary scientific literature. Examples may include textbooks, monographs, bibliographies, encyclopedias and reference books of all kinds which provide a summary of accepted knowledge about a topic or subject area in broad outline. Librarians disagree on the precision of this definition and some do not use the term at all, preferring categories of only primary and secondary sources.
What is "grey literature?" Grey literature is information that has not been formally published and, therefore, is unavailable or hard to find. A good analogy is “as a shadow which has not yet acquired substance”. Also called “fugitive literature,” it usually refers to knowledge that is out in the world but has not yet been formally written down and distributed. Researchers may discuss a research study with colleagues and references may turn up in that person's paper but the original study itself is not available. Sometimes the information in grey literature never becomes available to you! For example, if a pharmaceutical company is researching a new drug and doesn’t distribute the information outside of its own employees because it doesn’t want to help the competition.
Grey literature publications include theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, government documents, and reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.). (Alberani, 1990)
The Grey Literature Report of the New York Academy of Medicine notifies subscribers of grey literature publications in health services research and selected public health topics. You can search their Grey Literature Collection as if you were searching a library catalog. Records of results of interest would have to be retrieved like any other book. It also lists organizations that publish grey literature.