Systematic Reviews of Epidemiological Studies of Etiology and Prevalence

Systematic reviews and meta-analyses of observational, epidemiological studies are common. In this chapter, we focus on epidemiological studies of etiology and prevalence. We discuss the rationale for systematic reviews of such studies, highlighting fundamental differences between observational studies and randomized controlled trials (RCTs). We address the steps from shaping the research question, to defining the Population, Exposures, Comparators, and Outcomes (PECO) or Population and Condition (PC) in reviews of etiology or prevalence, to exploring heterogeneity and interpreting results. In contrast to high-quality RCTs, confounding and bias often distort the findings of epidemiological studies. Bigger is not necessarily better: smaller studies may devote more attention to characterizing populations, exposures, or conditions than larger studies. Indeed, there is a danger that meta-analyses of observational data produce precise but spurious results. A set of criteria should be developed, guided by general principles, to assess the risk of bias in different observational study designs. In the analysis and interpretation of observational studies, more is often gained by examining possible sources of heterogeneity between these studies’ results than by calculating overall estimates of relative risks or prevalences.