A measure of survival time for long-term follow-up studies of the elderly

Studies of determinants of longevity in the elderly by means of a follow-up study are usually based on samples with a wide age range and some selectivity. This poses the problem of constructing a measure of longevity that: (1) will make subjects in the sample comparable regarding survival time, independent of their age and sex; (2) is suitable for long-term follow-up studies; (3) can be used to check the absence or presence of selectivity of the sample with regard to longevity; (4) improves the comparability of studies with different designs and sampling schemes; (5) can take the mortality development over time into account. An individual measure of survival time is presented that satisfies these requirements: the realized probability of dying (RPD). Its construction is described. The RPD is derived from population life tables based on age, year of birth, and sex. For each subject, the relative position on the survival curve within the birth cohort is determined. In an illustration, the RPD is applied to data from a 28-year follow-up study of the elderly in the Netherlands. A comparison is made with other survival measures commonly used in this type of study. It is concluded that the RPD is a powerful and valid measure of longevity in elderly subjects, and that it can be useful in the study of determinants of longevity.