Comparative importance of the fertility model, the total fertility, the mean age and the standard deviation of age at childbearing in population projections

Using empirical fertility rates and population distributions, we study comparative contributions to births’ prediction errors of choices for the fertility model and of the approximation errors of three main fertility indicators (the total fertility, the mean and the standard deviation of age at birth, respectively: TFR, MAB, SDAB). Agreeing with theories of dynamic populations, we find high importance of accuracy of TFR and MAB. Yet, the role is limited in population projections of the estimates of SDAB and of the choice of the fertility model form. More attention may be paid in population projections to working out (interdependent) scenarios for TFR and MAB, while relaxing complexity of other aspects of fertility projection models. Our results suggest widening the uncertainty range for TFR in cases when the MAB projections are based on regressions on TFR or other simplified assumptions.