Using AND, OR, and NOT operators (also called Boolean Logic)
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Boolean searching is a technique to help researchers expand or restrict a search by using three language operators, AND, OR, NOT. Venn diagrams provide a pictorial representation of these concepts (see the Venn diagram below).
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Most research databases supply a search template with boxes which can be connected by one of the three operators to help you organize your search logic.
Use AND and OR to Combine Terms (or keywords, subject headings)
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Use AND to search for specific terms, usually retrieves fewer results.
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Use OR to search for synonyms or variations of terms, usually retrieves more search results (i.e.. expands a search).
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The operator NOT also narrows a search but can sometimes result in eliminating relevant items and should be used with caution.
beaver AND duck = platypus
source: http://i.imgur.com/8BFAq.png