Letters

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

EDITORIn 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

View larger version (15K): [in this window] [in a new window]   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% confidence interval (mmHg/100 mg dietary calcium)

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 blood pressure associated with a standard deviation difference in calcium intake) were taken to be regular regression coefficients (the difference in blood pressure associated with 100 mg difference in dietary calcium).³ Since the standard deviation of calcium intake is more than an order of magnitude less than 100 mg this led to the inclusion of erroneous data and to one of these studies taking over 99% of the weight of the meta-analysis of food frequency trials. Correcting the meta-analysis for this error (and several other mistakes) leads to a different picture (figure). There is no suggestion that the seemingly plausible explanation for differences between studies in which different dietary methodologies were used holds true.

Meta-analysis can distance readers from original data and leave them dependent on the care (or lack of care) taken by the meta-analysts. Plausible but spurious reasons for differences found between groups of trials can easily be generated. Had Cappuccio et al avoided the errors pointed out by Birkett, they might have produced an equally plausible explanation for differences in the opposite direction. They could have argued, for example, that food frequency questionnaires are less accurate than 24 hour recall, thus leading to weaker associations.

Examples of misleading meta-analyses of observational studies should not lead us to conclude that a return to subjective narrative reviews is warranted. Any worthwhile review should be systematic and employ strategies to avoid bias, but the statistical combination of studies is rarely appropriate in observational research. A clearer distinction is needed between systematic reviews and meta-analysis to prevent the former being discredited by poor versions of the latter.

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

1. Egger M, Schneider M, Davey Smith G. Spurious precision? Meta-analysis of observational studies. BMJ 1998; 316: 140-144[Free Full Text].
2. Davey Smith G, Phillips AN. Confounding in epidemiological studies: why "independent effects" may not be all they seem. BMJ 1992; 305: 757-759.
3. Birkett NJ. Comments on a meta-analysis of the relation between dietary calcium intake and blood pressure. Am J Epidemiol 1998; 148: 223-228[Abstract/Free Full Text].
4. Cappuccio FP, Elliott P, Allender PS, Pryer J, Follman DA, Cutler JA. Epidemiologic association between dietary calcium intake and blood pressure: a meta-analysis. Am J Epidemiol 1995; 142: 935-945[Abstract/Free Full Text].