Analysing capture-recapture data when some variables of heterogeneous catchability are not collected or asked in all registrations

Registrations in epidemiological studies suer from incompleteness, thus a general consensus is to
use capture–recapture models. Lately, there has been a thrust to incorporate covariates which relate
to the capture probabilities in order to improve the estimate of population size. Current approaches
utilize covariates observed in all the pertinent registrations. In epidemiological applications, one often
encounters capture–recapture data where some covariates are measured by some but not by all of the
registrations under scrutiny. In this article, the usefulness of these types of covariates is evaluated and
furthermore a methodology for analysing such capture–recapture data is presented. The methodology
is illustrated by applying it to data on neural tube defects in the Netherlands.