Boolean logic refers to relationships among search terms. Boolean operators limit results of a search by letting the user combine search terms or concepts. The three Boolean operators are the words "AND," "OR," and "NOT" and although PubMed states they should be capitalised, you may also retrieve the same citations not capitalising them.8 The AND operator is used to retrieve results that contain the entered search terms in a citation. A search for "stannous fluoride toothpaste AND sodium fluoride and potassium nitrate toothpaste" will retrieve only citations that reference BOTH. The AND is the default operator used in PubMed, so if a Boolean operator is not specified between two or more terms, PubMed will automatically use AND between terms.8
The OR operator looks for citations that have at least one of the terms and combines them together in one result. The OR operator is used to combine articles on similar topics or broaden your search. A search for "tooth erosion OR tooth wear" will retrieve citations that reference at least one of the terms. The OR Boolean operator is helpful to use when there are synonyms, e.g., tooth erosion OR erosive tooth wear. The NOT operator excludes the retrieval of terms from your search results. Typing "dental erosion NOT dental caries" excludes results about dental caries that also cause enamel damage. However, if articles refer to how enamel damage can occur, the NOT Boolean operator could eliminate relevant citations. Of the 3 Boolean operators, be cautious about using NOT.
PubMed automatically processes Boolean connectors in a left to right sequence, so when using multiple Boolean operators with more than two terms it may be important to enclose terms in parentheses, (tooth erosion OR tooth wear) AND (stannous fluoride toothpaste) AND (sodium fluoride and potassium nitrate toothpaste). The terms inside the parentheses, which is referred to as nesting, will be processed as a unit and then incorporated into the overall strategy.9
Boolean Operators can be used when typing in terms in the Search Box on the Home Page (Figure 1) and when using the Search Builder on the Advanced Search Page (Figure 6), which is accessed from the Home Page (found under the search box). Later you will see that Boolean Operators also can be used in performing a search using Clinical Queries.
Figure 6. Boolean Operators as Part of Search Builder.