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Boolean operators tell the database how to treat your search terms. So, searching for A AND B will return all results that contain both A and B. Searching A OR B will return results that contain either A or B or both. And searching A NOT B will return results that contain A but not B.
In addition, remember to put phrases in quotes, since a search for "child development" will return results containing the phrase "child development," where a search for child development (without quotes) will return results containing the word "child" and the word "development" anywhere in the text, not necessarily near each other.
You can also use wildcards to expand your search. The most common one is the asterisk. For example:
Searching for child* will return results containing the word child, but also the words children, childhood, childlike, and any other word that begins with "child."
Searching for freshm*n will return results containing both freshman and freshmen, as well as any other words that begin with "freshm" and end with "n."
Once you have some keywords in mind, use Boolean to turn those keywords into an effective search. For some tips and tricks on how to do that, check out this research guide.
You can also use this worksheet to keep track of your own keywords and search strings.
Check out the video below for an overview of how the AND, OR, and NOT operators work.