This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  Have all important and relevant studies been included?

Even if the choice of eligible study designs can be justified, it may be difficult to show that all relevant studies have been identified because of poor indexing and inconsistent use of study design labels by researchers. Comprehensive search strategies that focus only on the health condition and intervention of interest are likely to result in a very long list of citations including relatively few eligible studies; conversely, restrictive strategies will inevitably miss some eligible studies. In practice, available resources may make it impossible to process the results from a comprehensive search, especially since authors will often have to read full papers rather than abstracts to determine eligibility. The implications of using a more or less comprehensive search strategy are not known.