Systematic reviews and meta-analyses: when they are useful and when to be careful.

Systematic reviews and meta-analyses are increasingly popular study designs in clinical research. A systematic review is a summary of the medical literature that uses explicit and reproducible methods for searching the literature and critical appraisal of individual studies; in contrast, a meta-analysis is a mathematical synthesis of the results of these individual studies. These study designs can be useful tools for summarizing the increasing amount of knowledge that is gained from scientific papers on a certain topic. In addition, combining individual studies in a meta-analysis increases statistical power, resulting in more precise effect estimates. Although the specific methodology of systematic reviews includes steps to minimize bias in all stages of the process, investigators should be aware of potential biases such as poor quality of included studies, heterogeneity between studies, and the presence of publication and outcome reporting bias. This paper explains how systematic reviews and meta-analyses should be performed and how to interpret and implement their results. In addition, we discuss when meta-analyses are useful and when they are not.