Peer-reviewed articles written by authors who actually performed an original experiment or are recording their field observations, i.e. of organisms or medical patients. More on research articles from the Life Science Library.
Peer-reviewed articles written by authors who summarize or discuss trends in the primary literature. In the life science field, these are called "review" articles or "reviews". More on review articles from the Life Science Library.
Non-peer-reviewed articles, books, newspaper articles, encyclopedia entries, etc. (your background information likely falls into this category)
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