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To the Editor: Dr Lynskey and colleagues¹ used a matched cohort design to study the association between cannabis use by the age of 17 years with drug use later in life. Twins who had used cannabis by the age of 17 years were matched with their sibling who had not used cannabis by that age. This design eliminates possible confounding by any shared genetic and environmental factors. The authors estimated odds ratios (ORs) using conditional logistic regression.

Odds ratios tend to overestimate the magnitude of risk ratios when outcome events are common.² In Table 2 of the study by Lynskey et al, it appears that among 311 pairs of twins, both siblings went on to use cocaine in 61 pairs; only the cannabis user went on to use cocaine in 88 pairs; only the twin who had not used cannabis went on to use cocaine in 21 pairs; and neither . . . [Full Text of this Article]

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RE: "ESTIMATING THE RELATIVE RISK IN COHORT STUDIES AND CLINICAL TRIALS OF COMMON OUTCOMES"
Cummings
Am J Epidemiol 2004;159:213-213.
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