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In the sciences, primary (or empirical) research articles:
You may also choose to use some secondary sources (summaries or interpretations of original research) such as books (find these through the library catalog) or review articles (articles which organize and critically analyze the research of others on a topic). These secondary sources, particularly review articles, are often useful and easier-to-read summaries of research in an area. Additionally, you can use the listed references to find useful primary research articles.
(Adapted from "Scholarly Literature Types", Cornell University)