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To the Editor:
Veldink et al.1 reported a strong association between leisure time physical activity and earlier age at ALS onset and propose that this may reflect a possible adverse effect of physical activity. However, this association was based on analyses conducted among ALS cases only, and is thus potentially confounded by age (or, more specifically, birth cohort).
For example, it could be explained if younger individuals in the source population were more physically active than older individuals, as could be expected from secular trends in leisure time physical activity. It would be interesting to see results appropriately adjusted for age. A simple and direct way to do this would be to report: 1) the odds ratio (OR) for physical activity stratified by age; and 2) the results of a test for interaction between age and physical activity. The latter could be done by adding to the multivariate logistic …
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