THE GOMPERTZ EQUATION AS A PREDICTIVE TOOL IN DEMOGRAPHY

The Gompertz demographic model describes rates of aging and age-indepe ndent mortality with the parameters alpha and A, respectively. Estimat es of these parameters have traditionally been based on the assumption that mortality rates are constant over short to moderate time periods . This assumption is questionable even for very large samples assayed over short time intervals. In this article, we compare several methods for estimating the Gompertz parameters, including some that do not as sume constant mortality rates. A maximum likelihood method that does n ot assume constant mortality rates is shown to be best, based on the b ias and variance of the Gompertz parameter estimates. Moreover, we sho w how the Gompertz equation can then be used to predict mean longevity and the time of the n(th) percentile of mortality. Methods are also d eveloped that assign confidence intervals to such estimates. In some c ases, these statistics may be estimated accurately from only the early deaths of a large cohort, thus providing an opportunity to estimate l ongevity on long-lived organisms quickly.