Methodologic issues in systematic reviews and meta-analyses.

Systematic reviews of original research are increasing in number. Systematic reviews are distinct from narrative reviews because they address a specific clinical question, require a comprehensive literature search, use explicit selection criteria to identify relevant studies, assess the methodologic quality of included studies, explore differences among study results, and either qualitatively or quantitatively synthesize study results. Systematic reviews that quantitatively pool results of more than one study are called meta-analyses. Several organizations are collaboratively involved in producing high quality systematic reviews and meta-analyses. Familiarity with how to do a systematic review and meta-analysis will lead to greater skill in using this type of article. For clinicians, teachers, and investigators, systematic reviews and meta-analyses are useful sources of evidence.