Consider a case–control study in which prevalent cases of a given disease define the index
series and members of the base population without the disease are sampled to provide the
referent series. Information on a set of explanatory variables (eg, genotypes) is collected at
great cost for cases and controls. The objective of the study is to evaluate the relationship
between case status and the explanatory variables. Subsequently, an investigator notes that
the prevalence of a second disease was measured for the members of the index and …