title: Meta-analyses: what they can and cannot do.
abstract: Meta-analyses overcome the limitation of small sample sizes or rare outcomes by pooling results from a number of individual studies to generate a single best estimate. As long as a meta-analysis is not limited by poor quality of included trials, unexplainable heterogeneity and/or reporting bias of individual trials, meta-analyses can be instrumental in reliably demonstrating benefit or harm of an intervention when results of individual randomised controlled trials are conflicting or inconclusive. Therefore meta-analyses should be conducted as part of a systematic review, i.e., a systematic approach to answer a focused clinical question. Important features of a systematic review are a comprehensive, reproducible search for primary studies, selection of studies using clear and transparent eligibility criteria, standardised critical appraisal of studies for quality, and investigation of heterogeneity among included studies.  Cumulative meta-analysis may prevent delays in the introduction of effective treatments and may allow for early detection of harmful effects of interventions. As opposed to meta-analysis based on aggregate study data, individual patient data meta-analyses offer the advantage to use standardised criteria across trials and reliably investigate subgroup effects of interventions. Network meta-analysis allows the integration of data from direct and indirect comparisons in order to compare multiple treatments in a comprehensive analysis and determine the best treatment among several options.  We conclude that meta-analysis has become a popular, versatile, and powerful tool. If rigorously conducted as part of a systematic review, it is essential for evidence-based decision making in clinical practice as well as on the health policy level.
authors.affiliation: Basel Institute for Clinical Epidemiology and Biostatistics, University Hospital Basel, Switzerland.
authors.forename: Alain J
authors.initials: AJ
authors.lastname: Nordmann
authors.forename: Benjamin
authors.initials: B
authors.lastname: Kasenda
authors.forename: Matthias
authors.initials: M
authors.lastname: Briel
mesh_terms: D002318:Cardiovascular Diseases / Q000628:therapy*; D019317:Evidence-Based Medicine / Q000379:methods*; D058006:General Practice / Q000379:methods*; D006801:Humans; D015201:Meta-Analysis as Topic; D016032:Randomized Controlled Trials as Topic / Q000379:methods*