Systematic Reviews in Health Research

We begin with an introduction to systematic reviews and meta-analysis, discuss terminology and scope, provide historical background, and examine the potential, promise, and limitations of systematic reviews and meta-analysis. The statistical basis of meta-analysis begins with Laplace in the late eighteenth century, while efforts to summarize research for medical practitioners reach back to James Lind's 1753 “Treatise of the Scurvy.” Today, reviews have become essential tools for health care workers, researchers, consumers, and policymakers who want to keep up with the evidence they need. Systematic reviews and meta-analyses allow more objective appraisal of the evidence of traditional reviews and may resolve uncertainty when original research, reviews, and editorials disagree. A meta-analysis of randomized trials may enhance the precision of estimates of treatment effects and lead to the timely introduction of effective interventions. In contrast, meta-analyses of heterogeneous observational studies will often not be appropriate. Exploratory analyses, for example of subgroups of patients who are likely to respond particularly well to treatment (or the reverse), may generate promising new research questions to be addressed in future studies. Finally, systematic reviews may demonstrate the lack of adequate evidence and identify areas where further studies are needed.