Uses and abuses of meta-analysis.

Meta-analysis, the statistical combination of results from several studies to produce a single estimate of the effect of a treatment, continues to attract controversy. We illustrate the potentials and pitfalls of meta-analysis of controlled clinical trials. Cumulative meta-analysis demonstrates that this technique could prevent delays in the introduction of effective treatments. 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). We argue that meta-analysis should be performed only within the framework of systematic reviews--that is, reviews prepared using a systematic approach to minimise bias and address the combinability of studies.