meta-analysisSystematic reviews often include meta-analyses, in which the results of multiple studies are statistically combined to reveal the overall effect.

The type of meta-analysis and summary statistic depends on the type of data reported in the individual studies. Dichotomous outcomes (e.g. total mortality) are summarized using risk ratios, odds ratios, or risk differences, while continuous outcomes (e.g. growth, in mm) are summarized with mean differences or standardized mean differences. Other data that can be analyzed include ordinal outcomes, counts or rates of events and time-to-event data.

The summary statistics for each study are calculated, weighted according to various factors (sample sizes, precision of estimate and/or degree of bias), and used to calculate an overall summary statistic with confidence interval. The overall result indicates the statistical support for one treatment over the other. Output from meta-analyses are often presented as forest plots.

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