Education and debate

Systematic Reviews: Meta-analysis and its problems

Institute of Psychiatry, London SE5 8AF.

Including all relevant material - good, bad, and indifferent - in meta -analysis admits the subjective judgments that meta-analysis was designed to avoid. Several problems arise in meta-analysis: regressions are often non -linear; effects are often multivariate rather than univariate; coverage can be restricted; bad studies may be included; the data summarised may not be homogeneous; grouping different causal factors may lead to meaningless estimates of effects; and the theory-directed approach may obscure discrepancies. Meta-analysis may not be the one best method for studying the diversity of fields for which it has been used.

Why do we undertake systematic reviews of a given field? The most important reason is perhaps that we are concerned about a particular theory and wish to know how the evidence for and against stacks up. There are also practical reasons; single studies often use small numbers of subjects, and basing our estimates of effect . . . [Full text of this article]

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