Duration of disease, duration indicators, and estimation of the risk ratio

The authors investigated how duration of disease and duration of risk until onset of disease relate prevalence odds ratios and cumulative incidence ratios, respectively, to incidence density ratios in a steady-state population. Using information on durations, they demonstrated how incidence density ratios may be estimated from the information available in variants of the case-referent study and their cohort study analogues. The standard case-referent study involves an incidence series of cases and a prevalence series of noncases. One variant is a study of a prevalence series of cases and a prevalence series of noncases. Another variant involves comparison of two incidence series for sequential incident events. Factors associated with disease risk have been termed risk indicators, and, by analogy, factors associated with disease duration are defined as duration indicators. These duration indicators may be identified through the study of duration ratios. Ratios of durations in steady-state populations may be combined with the prevalence and cumulative incidence information available in the variants of the standard study designs to obtain estimates of incidence density ratios.