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Basic Research Process

Using Boolean Operators

Boolean operators are logical connectors between words. The most used are AND, OR, and NOT. They are used to combine or exclude search terms (keywords) in a search string to filter your search results.

Some search engines require Boolean operators to be typed in all capital letters, so it is a good habit to always capitalize them. These terms may be represented in various ways depending on the database. Some databases even allow the plus (+) and minus (-) signs for AND and NOT.  

Check the help section in any database to see if these operators are represented differently.

AND is used to find the resources in a database that contain two or more of your terms. This can narrow down the number of results you find to those more relevant, thus focusing your search.

This is the process of finding only the items where two ideas overlap, as in the illustration below using AND.

Conceptual Venn diagram of two overlapping concepts, represented by red and blue circles and an overlapping purple area representing the area of overlap

The search represented below finds only those individual resources which have both C AND A.

Represents a group of circles, each with various groups of the letters a, b and c, filtered through a search to give a resulting set with only those circles containing both C and A

You can combine several terms with operators. In the example above, if you search for AND AND C, that would return one result (A, B, C).

NOT is used to find resources that do not contain terms that are not directly relate to your topic. This also narrows down the number of results you find to those more relevant, further focusing your search.

In this process one is finding items where two concepts do not overlap, as in the illustration below using NOT.

a Venn diagram illustrating eleminating one concept from a search using the Boolean operator NOT

The search represented below finds only those individual resources which have C NOT A.

You can combine logical operators. In this example if you searched for AND NOT A, that would return one result (C, B).

OR is used to make groups of two or more similar terms. This increases the number of relevant results you find by increasing the number of words for the same concept, expanding your search. This is especially good when you get few results.

Including two or more similar terms for concepts in your topic, using OR, increases your chances of finding useful information.

Note that OR also returns those terms together.

Venn diagram showing that using OR with two options includes both individually and the two terms together

The search represented below returns those individual resources which contain either C OR A, including both.

CAUTION: When using an OR group in a search with other logical operators always put parentheses around the OR group. See the Combinations tab.

The Boolean operators may be combined. BUT, just like in high school algebra, the order in which they are used is important.

A database first evaluates all the AND & NOT operators. After those are evaluated then OR is the last operator applied.

ALWAYS (put parentheses around a group of terms joined by OR) then the database treats them like one search term.

To combine terms with an (OR group), use AND plus the other term(s) after the close parentheses.

NOTE: This search returns Blue and Yellow; Red and Yellow; along with Red & Blue & Yellow (because OR will return both terms together). But not yellow alone.

To exclude any terms from searching with an (OR group) follow the close parentheses with NOT plus the term you wish to exclude.

NOTE: This search returns Blue ; Red ; including Red & Blue together (because OR will return both terms together). But, NOT those items where the group includes Yellow. 

Use parentheses around all terms combined using OR, to treat them as one group.

Boolean Tutorial

Advanced Search Example Using Boolean Operators

This is an example of a search. Let's say you are researching how childhood obesity correlates with academic performance in elementary school children.

1) Putting childhood obesity in quotation marks (" ") will guarantee the search results to include this phrase.

2) Putting OR between these related keywords will broaden your search to related topics on academic performance.

3) Putting OR between elementary and primary makes sure you do not miss results using this synonym.

4) Specifying NOT to exclude high school or secondary education allows you to make sure you're only getting sources for the age group specified by your question.