Immigration bias in cohort fertility

Felix Roessger, Max Planck Institute for Demographic Research

In this paper we examine the effects of immigration on the cohort total fertility rate (CTFR) as conventional measure for completed cohort fertility. As almost all conventional demographic measures it was designed for “closed” populations and can be consequently biased, if it is applied to an “open” population. We show that this bias arises from the fact that the immigrant population is not included in the population under exposure properly. Consequently, we propose a measure adapted for immigration, which treats migrants as if they always had been in the considered population. However, this new measure is restricted due to its high requirements on detailed fertility data. For this reason, a further iterative procedure is examined, which models the relevant process under certain assumptions but with less data requirements.