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Searching the Scientific Literature


Using the Boolean operators (AND, OR, NOT) to combine keywords in a database search allows you to narrow or expand your search. To build a complex search using two or more Boolean operators, you will need to learn the advanced technique of nesting (see Creating a Search Query for more details).

In the Venn diagrams below, the area retrieved is the yellow (or most lightly shaded) area.


Use AND to narrow a search. BOTH terms must be present in any references you retrieve.

Example: global warming AND forests 


Use OR to expand a search. Your search will retrieve records with EITHER of the terms.

Example: ethanol OR methanol 

OR is most often used to combine synonyms or like terms.



Use NOT to exclude a term. Records with the first term will be retrieved, but any records with the second term will be eliminated.

Example: california NOT baja