Boolean searching is a technique of searching by linking concepts together with commands known as Boolean operators. Examples of Boolean operators are "AND" and "OR." Boolean searching is a powerful tool for refining a search to produce a highly focused result.
Let's say, for example, that we want information about sports-related injuries. We could search for the phrase, "sports-related injuries," but that would leave out documents that say "injuries due to sports." Instead, we may use the Boolean operator "AND" to link together the concepts "sports" and "injuries." This could be expressed in a diagram:
In the illustration, the left circle represents all the documents which contain the word "sports," and the right circle represents all the documents which contain the word "injuries." Documents which contain both words are represented by the area of overlap where the two circles intersect. The search "sports AND injuries" would retrieve documents which contain both words.
The "AND" operator is the most commonly used one, because it makes the resulting set smaller and more focused. Another operator which makes the resulting set smaller is "NOT." "NOT" excludes a term from the resulting list. "Sports NOT golf," for example, subtracts all "sports" documents that include the word "golf." We might retrieve documents about the Olympics, football, tennis, or curling, but none of the documents in the resulting set would contain the word, "golf."
An operator which makes the resulting set larger is "OR." When we searched for the word "injuries," we might have missed documents which described "wounds" or "trauma" without using the word "injuries." To find all of these we might search for "injury OR trauma OR wounds." "OR" broadens the resulting set to include documents which contain at least one of these words. In the diagram below, all of the sets represented by circles would be part of the result set.
Boolean statements may be combined into a single search string. Parentheses are used to "nest" one Boolean expression within another and thereby govern the order in which the operations take place. In the following example,
- sports AND injuries OR accidents
- (sports AND injuries) OR accidents
- (injuries OR accidents) AND sports
- sports AND (injuries OR accidents)
- (sports and injuries) OR (sports AND accidents)
the first two expressions are identical: "sports" AND "injuries" are combined first, and the resulting set is combined with "accidents". In each case the number of documents in the result list is the same.
In the third expression, "(injuries OR accidents) AND sports ," the words within the parentheses, "injuries" and "accidents," are combined using the operator OR, and then combined with "sports" using AND. Expression number 4 is equivalent, and would find the same number of documents as number 3.
Expression number 5, "(sports and injuries) OR (sports AND accidents)," is equivalent to numbers 3 and 4. The difference between the first two examples and the last three is that numbers 3,4, and 5 find only about a tenth as many articles, because all the articles must contain the key word "sports." In the first two expressions, only the articles which contain the word "injuries" must contain "sports". The list resulting from number 1 (or number 2) also contains all the articles including the word "accidents," whether or not "sports" appears.
The operators listed above are described as they are used in EBSCOhost. Some other databases may use symbols rather than words, e.g., "&" instead of "AND," or they may call the operators by different names, although the function is the same.
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Updated: 6 February 2012