Systematic reviews of the literature: an introduction to current methods

Systematic reviews are a type of evidence synthesis in which authors develop explicit eligibility criteria, collect all the available studies that meet these criteria, and summarize results using reproducible methods that minimize biases and errors. Systematic reviews serve different purposes and use a different methodology than other types of evidence synthesis such as narrative reviews, scoping reviews, and overviews of reviews. Systematic reviews can address questions regarding effects of interventions or exposures, diagnostic properties of tests, and prevalence or prognosis of diseases. All rigorous systematic reviews have common processes that include (1) determining the question and eligibility criteria, including a priori specification of subgroup hypotheses, (2) searching for evidence and selecting studies, (3) abstracting data and assessing risk of bias of the included studies, (4) summarizing the data for each outcome of interest, whenever possible using meta-analyses, and (5) assessing the certainty of the evidence and drawing conclusions. There are several tools that can guide and facilitate the systematic review process, but methodological and content expertise are always necessary.