Ultimate fertility levels: a modified projection method for low fertility countries

Recently, the United Nations Population Division adopted a new method for projecting total fertility (TF) for all countries. The new projection method was well received but raised discussion about the model assumption that in the long run, the TF will oscillate around the approximate replacement level of 2.1 for all countries. In this paper, we investigate a modified TF projection model, whereby the ultimate fertility levels are country-specific and estimated using a Bayesian hierarchical model. Expert opinion is incorporated into the model by setting the upper bound on the ultimate fertility level to 2.1. Under the proposed model, ultimate fertility levels are smaller though within 0.25 child of the current UN projection for most low fertility countries, and 1.9 (80% projection interval 1.6-2.3) for countries that have not yet completed the fertility transition, compared to 2.1 (1.8-2.4) for the existing method.