Meta-analysis is a statistical technique for combining data from independent studies to produce a single estimate of effect and associated uncertainty around this estimate. In the context of health technology assessments, source studies are usually randomised controlled trials (RCTs). Meta-analysis can be used whenever there is more than one study that has estimated the effect of an intervention (or risk factor) using the same outcome measure, and source studies are sufficiently similar in terms of the participants, interventions compared, settings, duration and definition and measurement of the outcome measure. In this way, it will be reasonable to combine the results of these studies. This can be assessed qualitatively in a systematic review prior to the quantitative synthesis, and heterogeneity in the results of the source trials can be tested statistically. Results of meta-analyses are usually reported based on fixed and random effects modelling, and results are displayed graphically using a Forest plot.
The PRISMA Checklist has been developed to guide reporting of meta-analyses more broadly. Additionally, a systematic approach to undertaking meta-analyses of RCTs in many areas of healthcare has been co-ordinated by the Cochrane collaboration. More recently, meta-analysis techniques have been further developed to support comparisons where head to head trials (i.e. direct evidence) is lacking (see indirect treatment comparison for more details).
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