Meta-analysis: weighing the evidence.

Use of meta-analytical (quantitative overview) techniques is now commonplace in a wide range of medical research contexts, with a rapid rise in its frequency of use being particularly apparent in the last decade. The history of meta-analyses in other fields, particularly psychology and educational research, is longer. Many methods have been proposed and used, from crude 'vote counting' of studies showing significant or non-significant results, through methods for combination of effect size estimates based on fixed or random effects models, to general linear mixed models and Bayesian methods. The history of meta-analysis and the advantages and disadvantages of various approaches to it are briefly reviewed in this paper, with reference to pharmaceutical product licence applications, other reviews of clinical trials and epidemiological studies, and health services research.