Boolean operators are words such as and, or, and not that you use to combine search terms. An AND search says, find me articles that contain both of my search terms. Searches for cats AND dogs will retrieve articles that contain both the word "cats" and the word "dogs." If an article only contains the word "cats" but not the word "dogs", that article will not be retrieved. 
Cats AND Dogs
Want to know more about creating effective searches using boolean operators? Follow the link below for information from MIT Libraries.
An OR search says, "find me anything that mentions either this term or that term." Searches for cats OR dogs will retrieve all the articles that contain the word "cats", the word "dogs", or both. You can also add more search terms into the mix, with the understanding that this will further expand your search.
Cats OR Dogs
A NOT search says, "Find me anything that mentions this term but not that one." Searches for cats NOT dogs will retrieve all the articles that contain the word "cats", but excludes from that list anything that uses the word "dogs." You have to be very careful when using this operator, as it will severely restrict your results and you may miss something important. An example of when to use NOT might be when you're looking for articles about dolphins (the animal) and you keep getting results with Dolphins (the Miami football team) You could do a search like, dolphins NOT football.
Cats NOT Dogs
If you got too few results try one or more of these tips:
Use an OR with synonyms to broaden your search
Consult a thesaurus or index for more search terms
Reduce the number of concepts
Use more general terms
Use truncation to get variations or alternate spellings of your term
Use the broadest search possible (usually Keywords in most databases)
Check your spelling
Check to see if you used AND and OR properly
Check to see if you are using the best database for your topic
If you get too many results try one or more of these tips:
Use an AND to narrow your search
Use more precise terms
Use subject headings instead of Keywords
Consult a thesaurus or index for more narrower terms
Remove truncation
Search in a specific field, such as the Subject or Title field, instead of searching Keywords
Use limits such as language, years or document type.
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