BMJ 1999;318:56 ( 2 January )

Letters

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

The first 150 words of the full text of this article appear below.

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.2 Birkett's critique of one of our examples illustrates these points.3

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Cappuccio et al showed a weak inverse association between calcium intake and blood pressure.4 Stratified analysis showed that the studies in which food frequency questionnaires were used showed a much greater association than the studies in which diet history or 24 hour dietary recall were used (figure, top). Cappuccio et al argued that this could be expected since food frequency questionnaires assess habitual diet and long term calcium intake was likely to be the important factor influencing blood pressure.

Birkett showed that for one study included in the meta-analysis standardised regression coefficients (the difference in . . . [Full text of this article]
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Rapid Responses:

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Misleading inaccuracies do more harm than good
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bmj.com, 13 Jan 1999 [Full text]
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