"Boolean operators" are the connector words that are used to combine sets of search results to achieve certain desired effects. They are named for George Boole, a 19th century English mathematician. The most commonly used Boolean operators are AND, OR, and NOT.

Here is a handy "rule of thumb" for the Boolean operators AND and OR:

• Use AND when you want to find articles that contain ALL terms/concepts. Combining sets with AND will produce a smaller set, since all items in the set must contain all terms.
• Use OR when you want to find articles that have ANY of the terms/concepts. Combining sets with OR will produce a larger set ("OR makes more"), since the items in the set must contain only one of the terms.

For more information about Boolean operators, check out these resources:

• this link from U Mass Boston, which includes a 2-minute video

Well, this can be a problem indeed! Here are some tactics to try:

• check your spelling!
• remove one or more terms from your search query
• search on related terms or synonyms, combining terms using the Boolean operator OR to make a larger set
• substitute a more general term
• remove filters, or apply them incrementally
• when available, use the Explode feature to search on narrower terms as well as the more general term

This can be as much of a problem as getting too few results. Try one of these strategies to simultaneously reduce the numbers of hits, and focus the results:

• add another search term, using the Boolean operator AND to make a smaller set
• search on a more specific term
• in databases that offer this feature, such as MEDLINE, restrict your search to MeSH Major Topic
• in databases that offer this feature, add one or more subheadings
• apply one or more filters, such as publication date, article type, or age group
• when available, deselect the Explode feature, and search only on the more general term, or choose a more specific term on which to search