A SEMIPARAMETRIC ESTIMATE OF EXTRA-POISSON VARIATION FOR VITAL-RATES

We introduce a method for estimating overdispersion in Poisson models for vital rates, We assume smoothness conditions on the counts to obta in pointwise variance estimates that we combine to obtain an estimate of the overdispersion parameter. We create confidence intervals about the observed rates using this estimate and an approximation based on t he gamma distribution. The advantage of this method is that the estima tes of the superpopulation rates do not depend on the smoothness assum ption, yet when this assumption is met we obtain approximately unbiase d estimates of the overdispersion parameter. Thus, we may calculate co nfidence intervals for vital rates under an overdispersed Poisson mode l without making parametric assumptions on the mean rates. (C) 1997 by John Wiley & Sons, Ltd.