6.4.7  Boolean operators (AND, OR and NOT)

A search strategy should build up the controlled vocabulary terms, text words, synonyms and related terms for each concept at a time, joining together each of the terms within each concept with the Boolean ‘OR’ operator: see demonstration search strategy (Figure 6.4.a). This means articles will be retrieved that contain at least one of these search terms. Sets of terms should usually be developed for the healthcare condition, intervention(s) and study design. These three sets of terms can then be joined together with the ‘AND’ operator. This final step of joining the three sets with the ‘AND’ operator limits the retrieved set to articles of the appropriate study design that address both the health condition of interest and the intervention(s) to be evaluated. A note of caution about this approach is warranted however: if an article does not contain at least one term from each of the three sets, it will not be identified. For example, if an index term has not been added to the record for the intervention and the intervention is not mentioned in the title and abstract, the article would be missed. A possible remedy is to omit one of the three sets of terms and decide which records to check on the basis of the number retrieved and the time available to check them. The ‘NOT’ operator should be avoided where possible to avoid the danger of inadvertently removing from the search set records that are relevant. For example, when searching for records indexed as female, ‘NOT male’ would remove any record that was about both males and females.


Searches for Cochrane reviews can be extremely long, often including over 100 search statements. It can be tedious to type in the combinations of these search sets, for example as ‘#1 OR #2 OR #3 OR #4 …. OR #100’. Some service providers offer alternatives to this. For example, in Ovid it is possible to combine sets using the syntax ‘or/1-100’. For those service providers where this is not possible, including The Cochrane Library for searches of CENTRAL, it has been recommended that the search string above could be typed in full and saved, for example, as a Word document and the requisite number of combinations copied and pasted into the search as required. Having typed the string with the # symbols as above, a second string can be generated by globally replacing the # symbol with nothing to create the string ‘1 OR 2 OR 3 OR 4 …. OR 100’ to be used for those service providers where the search interface does not use the # symbol.