Population estimation using the multiple system estimator in the presence of continuous covariates

In the presence of continuous covariates, standard capture-recapture methods assume either that the registrations operate independently at the individual level or that the covariates can be stratified and log-linear models fitted, permitting the modelling of dependence between data sources. This article introduces an approach where direct dependence between registrations is modelled leaving the continuous covariates in their measurement scale. Simulations show that not accounting for possible dependence between registrations results in biased estimation of both the population size and standard error. The proposed method is applied to Dutch neural tube defect registration data.