Boolean operators (AND/OR/NOT)
Boolean operators (AND/OR/NOT)
Once you have identified search keywords and searched for each keyword individually, you can combine these using Boolean operators. Boolean operators are conjunctions that can be used in many search tools to either expand or narrow your search. The name is derived from the English mathematician George Boole. There are 3 different operators: AND, OR, and NOT.
We recommend that you always search each keyword individually. This helps you keep track of what each keyword contributes to your search in terms of references, and you can then eliminate keywords from your search if they contribute an unusually large number of irrelevant references. Then, combine the keywords you have defined with the respective Boolean operators.
Explanation of Boolean Operators
OR
An OR-search will return references with content that includes any of the search keywords, or both. All keywords, controlled or free search keywords that are identified under one main element should be combined with OR. In other words, we use OR to combine keywords that are synonyms or nearly synonyms. This is to expand your search and find references that deal with your main element, regardless of which keywords are used in the reference. The more keywords combined with OR, the more references we will find.
AND
An AND-search will find references with content common to both search keywords or main elements. Once you have found all relevant search keywords for each main element and combined them with OR within each individual main element box, you finally combine the result from each main element box with AND. This narrows the search to references where all main elements are represented. The more main elements are combined with AND, the fewer references you will find.
NOT
A NOT-search will exclude references with specific search keywords from your search results. We recommend that you use NOT cautiously, as there are usually better search techniques available to adjust your literature search!
For example, if you only want to find studies that deal with humans, and thus use NOT ‘animals’ in your search, you exclude all studies where the word ‘animals’ is used. You then exclude studies that deal with both humans and animals, and you may miss out on relevant articles.
Note: All databases use these Boolean operators, but the syntax for NOT can vary from database to database.
Rule of thumb
Inside the box for each main concept, always use OR. Between the boxes for the different main concepts, always use AND!
See example below:
Deviations from this rule of thumb occur, but always remember to have full control over all steps in the search process.
