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How to search in the Health Sciences

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Boolean Operators - Overview

Any time you combine more than one concept in a search you need to consider using boolean operators.   

  • Databases require mathematical rules in order to function
  • These mathematical functions are known as Boolean Operators
  • There are different operators, the most commonly used are AND, OR, NOT
  • Boolean operators allow for the combination of concepts in databases
    • OR expands a search
    • AND focuses a search
    • NOT removes from a search

OR is used to expand a concept

When searching you sometimes need to think of synonyms for a concept in order to capture all the information on that concept.

  • For example, to capture all the information on heart attacks you would search:
    • heart attack [1] OR myocardial infarction [2] OR cardiac arrest [3]
  • The Venn diagram below illustrates how 3 sets of information, combined using OR, gives you all three sets of information in your results

What to watch for: do not overdo it when it comes to synonyms, only use the most logical; do not include broader concept terms, this will take away the focus of your search.

Venn Diagram showing intersection of three circles containing heart attack, myocardial infarction, cardiac arrest

AND is used to focus a search

AND is used in searching when you want the results to contain all the concepts important to your research

  • For example, to capture information on the use of exercise by an elderly population as part of a fall prevention program you would search exercise AND elderly AND fall prevention
  • The Venn diagram below illustrates how 3 sets of information, combined using AND, would give you one set of information: (exercise [1], elderly [2] , fall prevention [3]); only the small centre set of information would be in your results, the one that contains all three elements

What to watch for: be careful of having too many concepts, if you AND too many ideas together you will end up with zero results

Venn Diagram showing three circles including exercise, elderly, fall prevention and pointing out the intersection where all three overlap


NOT excludes words from your search

  • NOT narrows your search, telling the database to ignore concepts that may be implied by your search terms
  • For example:  if you were searching for articles on nursing and education classes but you did not want articles on breast feeding would search: (nursing [1] AND education classes[2]) NOT breastfeeding[3] 
  • The Venn diagram below illustrates how you would use NOT to exclude information from a search, you would retrieve information that contains (nursing [1] and education classes [2]), the arrow head shaped set in the middle; the set (nursing [1], education classes [2], breastfeeding [3]) would be excluded from the results

What to watch for: be careful using NOT as you may inadvertently exclude helpful articles from your search

Venn Diagram showing three circles with arrow indicating that the intersection between nursing and education that doesn't contain breast feeding is the resulting set

Rules for combining concepts using Boolean Operators

  • Databases follow the commands you type in and return results based on those commands.
  • Databases follow a specific logical order when using Boolean operators: 
    • Databases usually recognize AND as the primary operator, and will connect concepts with AND together first.
    • If you use a combination of AND and OR operators in a search, you must enclose the words to be "ORed" together in brackets so the database will combine those as a set before combining the AND concepts together.
  • Examples:
    • ethics AND (cloning OR reproductive techniques)
    • (ethics OR morals) AND (bioengineering OR cloning)
    • (nursing AND education classes) NOT (breast feeding)