To the Editor.—
In favorable circumstances, a randomized, controlled intervention trial provides a test of the hypothesis that a "risk factor" is a causal factor. It is less widely recognized that analysis of the following kinds of evidence provides further tests: (1) sex- and age-specific death rates for coronary heart disease (CHD), (2) temporal trends in point 1, (3) mortality ratios by age and sex, (4) sex and age prevalence of persons with the risk factor, and (5) where available, temporal trends in point 4. Without going into tedious detail, it will be readily appreciated that if a risk factor is a causal factor, then changes in its level with age and time or both must manifest in the sex- and age-specific death rates for CHD. Exploiting the previously mentioned kinds of evidence, our problem is to discern whether the perturbations of death rates predicted by the causal hypothesis are
Burch PRJ. The Multiple Risk Factor Intervention Trial. JAMA. 1983;249(11):1435. doi:10.1001/jama.1983.03330350017009
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