Quantifying lifetime risk of psychiatric disorder

Lifetime risk for a particular psychiatric disorder is the proportion of a given population that would develop that disorder if no unaffected individual died prior to some specified age. Since it is impossible to quantify risk beyond some normal or maximum lifespan, lifetime risk can only be meaningfully considered “as of some particular age t”. In this paper lifetime risk to age t (LTR_t) is defined in terms of a hazard function and its relationship to other possible measures of risk is examined. The commonlly used morbidity risk ratio (MRR_t) is shown to differ from LTR_t; the value of MRR_t depends not only on the hazard function for onset of the disorder of interest but also on mortality among unaffected individuals. LTR_t, MRR_t and two other measures of risk are also compared in terms of the nature of the sample data that are required in order to estimate them. A numerical example illustrating estimation procedures is discussed in detail.