Meta-analyses of observational data should be done with due care

doi:10.1136/bmj.318.7175.56

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George Davey Smith, Professor of clinical epidemiology,
Matthias Egger, Senior lecturer in epidemiology

Department of Epidemiology and Public Health Medicine, University of Bristol, Bristol BS8 2PR

EDITOR—In our review of meta-analyses of observational studies we pointed out that all these are susceptible to all the biases inherent in observational research1 and that it is easy to generate seemingly plausible explanations for findings of observational studies that are in fact spurious.2Birkett's critique of one of our examples illustrates these points.3

Relation between dietary calcium and systolic blood pressure by method of dietary assessment. Erroneous data published by Cappuccio et al4 (panel A) and corrected analyses by Birkett3 (panel B). Slopes with 95% …