Smoking as “Independent” Risk Factor for Suicide: Illustration of an Artifact From Observational Epidemiology?

2 widely used criteria for determining whether an association between a risk factor & a disease is causal are dose response & independence from other factors. Data from a large US risk factor study (MRFIT) show a relationship between cigarette smoking & suicide that meets these criteria, yet appears to be biologically implausible. It is likely that many more such associations, for other exposures & other diseases, are equally spurious, but are protected by their lack of obvious implausibility.