“Spurious Precision? Meta-Analysis of Observational Studies”, 1998-01-10 (; backlinks; similar):
In previous articles we have focused on the potentials, principles, and pitfalls of meta-analysis of randomized controlled trials. Meta-analysis of observational data is, however, also becoming common. In a MEDLINE search we identified 566 articles (excluding those published as letters) published in 1995 and indexed with the medical subject heading (MeSH) term “meta-analysis.” We randomly selected 100 of these articles and examined them further. Sixty articles reported on actual meta-analyses, and 40 were methodological papers, editorials, and traditional reviews (1). Among the meta-analyses, about half were based on observational studies, mainly cohort and case-control studies of medical interventions or aetiological associations.
Summary points:
Meta-analysis of observational studies is as common as Meta-analysis of controlled trials
Confounding and selection bias often distort the findings from observational studies
There is a danger that meta-analyses of observational data produce very precise but equally spurious results
The statistical combination of data should therefore not be a prominent component of reviews of observational studies
More is gained by carefully examining possible sources of heterogeneity between the results from observational studies
Reviews of any type of research and data should use a systematic approach, which is documented in a materials and methods section