Protopathic bias is the type of bias (also called as reverse-causality) that is a consequence of differential misclassification of the exposure related to timing of occurrence. It occurs when a change in exposure taking place in the time following disease occurrence is the incorrectly thought to precede disease occurrence. For instance, a ?nding that alcohol has a protective effect for the clinical gallstone disease may be explained by the reduction in alcohol use because of the symptoms related to gallstone disease.