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Boolean operators--OR, AND, NOT--are used between search terms to combine concepts in a more precise way than is possible with keyword searching.

AND
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Venn diagram with the overlapping section shaded to show the boolean AND
The AND operator narrows a search, retrieving only records containing both term(s). AND is used to narrow a topic to a specific aspect.

Example: anorexia AND "college students"
OR
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Venn diagram with all sections shaded to show the boolean OR
The OR operator broadens a search, retrieving records that contain either term, but not necessarily both. It is used to combine synonyms or related concepts.

Example: anorexia OR bulimia
NOT
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Venn diagram with only one section shaded to show the boolean NOT
The NOT operator also narrows a search by removing all records that contain a particular word or phrase.

Example: anorexia NOT bulimia

The NOT operator is rarely used. It often screens out useful records.