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Systematic reviews and meta-analysis 

Systematic reviews and meta-analysis
Systematic reviews and meta-analysis

Matthias Egger

, George Davey Smith

, and Jonathan Sterne

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date: 16 May 2022

Systematic reviews are ‘studies of studies’ that are done using a systematic approach to minimize bias and random error. Similar to other research, the problem to be addressed and the collection and analysis of the data should be detailed in a study protocol. This should include eligibility criteria for studies to be included, a comprehensive search strategy for such studies, and an assessment of their methodological quality. Systematic reviews may, or may not, include meta-analysis, a statistical combination of results from several studies to produce a single estimate of the effect of an intervention. Systematic reviews allow for a more objective appraisal of the evidence than traditional, narrative reviews and may contribute to resolve uncertainty and identify areas where further studies are needed. Meta-analysis, if appropriate, will enhance the precision of estimates of intervention effects, leading to reduced probability of false negative results, and potentially to a timelier introduction of effective interventions. Meta-analyses are, however, liable to numerous biases both at the level of the individual trial (‘garbage in, garbage out’) and the dissemination of trial results (publication bias and other reporting biases). Meta-analysis should be performed only within the framework of carefully conducted systematic reviews. The thoughtful consideration of heterogeneity between study results is an important aspect of systematic reviews and meta-analyses, and particularly important in meta-analyses of observational studies.

The volume of data that need to be considered by practitioners and researchers is constantly expanding. In many areas it has become simply impossible for the individual to read, critically evaluate and synthesize the state of current knowledge, let alone keep updating this on a regular basis. Reviews have become essential tools for anybody who wants to keep up with the new evidence that is accumulating in his or her field of interest. However, since Mulrow (Mulrow 1987) drew attention to the poor quality of narrative review articles in the 1980s, it has become clear that conventional reviews are an unreliable source of information. Since then there has been increasing focus on formal methods of systematically reviewing studies, to produce explicitly formulated, reproducible, and up-to-date summaries of the effects of healthcare interventions. This is illustrated by the sharp increase in the number of reviews that used formal methods to synthesize evidence (Fig. 6.14.1). This chapter discusses terminology and scope, provides some historical background, and examines the potentials and pitfalls of systematic reviews and meta-analysis.

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