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To the Editor:
A recent paper by Racette et al.1 reported that “The estimated prevalence of parkinsonism was higher within a sample of male Alabama welders vs the general population of male residents of Copiah County, MS.” The authors mention potential reviewer bias and a proxy for age stratification as caveats. These and other threats to valid causal inference, such as multiple testing biases from selection of occupational codes counted as “welders,” can potentially create significant statistical associations between welding and Parkinson disease (PD), even if welding does not cause increased PD risk.
To test whether a study provides evidence that a statistical association is causal, it is common to refute plausible rival (non-causal) hypotheses,2 especially confounding and biases. For example, if older people are both more likely to have PD and to be welders, as shown in the following causal graph3:
PD ← Age → Exposure1 this provides an alternative to the causal hypothesis …
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