Regional probabilistic fertility forecasting by modeling between-country correlations

The United Nations Population Division releases country fertility estimates and projections every two years, currently using the model of Alkema et al (2011, Demography) for total fertility rate (TFR). This Bayesian hierarchical model produces a predictive distribution of TFR for each country. We extend this model to allow probabilistic projection of the TFR for any set of countries, such as a region or trading bloc. We model the correlation between country TFRs that is not captured by the original model as a linear function of time invariant covariates, namely whether the countries are contiguous, whether they had a common colonizer after 1945, and whether they are in the same UN region. This correlation structure is incorporated into the original model's error distribution and is shown to improve the calibration of predictive intervals for the future TFR of regions.