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Assistive Technology

Boolean Operators

The databases will often not understand your query if you enter it as a natural language sentence, such as your full research or PICO question.  Instead, you want to use the search terms that you brainstormed to create an advanced search strategy.  By using Boolean Operators, you can tell the databases precisely how you want your search terms to be searched. 


  • Limits your search to articles that contain both of the search terms
  • Narrows or focuses your search; you will typically get fewer results the more ANDs you use
  • Use AND between different concepts
  • For example: dental anxiety AND music therapy
                         Boolean And Diagram


  • Expands your search to articles that contain either of the search terms
  • Broadens your search; you will typically get more results the more ORs you use
  • Use OR between different search terms for the same concept
  • For example: dental anxiety OR dental fear OR dental phobia
                         Boolean Or Diagram


  • Excludes a term from your search
  • Narrows or focuses your search; you will typically get fewer results the more NOTs you use
  • For example: music therapy NOT art therapy
  • BE CAREFUL!  It is very easy to exclude too much and accidentally miss important and relevant literature.
  • Try to use combinations of AND and OR, then exclude results yourself, instead of using the Boolean NOT
                         Boolean Not Diagram

Understanding Boolean Operators

Video created by McMaster Libraries.  This video is licensed under a Creative Commons Attribution 4.0 International License.


Lastly, when creating a more complicated or advanced search, you can use parentheses to group your keywords together and tell the database precisely how you want the terms searched.  The database will perform the searches within parentheses before the searches outside of parentheses.  This is similar to the way parentheses are used in math.

Use parentheses any time you have more than one keyword for a particular concept.  In other words, when you are using the boolean operator OR, put parentheses around all of the OR'd terms.

For example: (dental anxiety OR dental fear OR dental phobia) AND (music therapy OR music)