Consistency by itself proves nothing; if thirty studies each ignore the same confounder, all can easily be biased. Strength of association is vulnerable for the same reason; as pointed out earlier, children’s shoe sizes are strongly associated with but not causally related to their reading aptitude. Specificity has always been a particularly controversial criterion. It makes sense in the context of infectious disease, where one agent typically produces one illness, but less so in the context of environmental exposure. Smoking leads to an increased risk of a variety of other diseases, such as
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