Comparing Forecast Methods for Birth-Order Cohort Fertility with an Application to Japan

In a cohort approach, the extrapolation may refer either to the incomplete fertility of a cohort or to the time series of the parameters of the model. By "forecasting" or "extrapolation" we mean here the capacity of a method to both complete the childbearing period of cohorts of women who have not yet reached its end and forecast the entire set of fertility rates for future cohorts, using the parameters of the model. Various methods have been proposed in the literature but, to our knowledge, there is no study in the international literature which investigates the performances of the various models for the extrapolation of cohort fertility, especially on both perspectives. Yet, this is of particular relevance for projections, especially for those with a long time horizon. This study intends to fill in this gap, comparing the pros and cons of the models listed above using Japan as case study. In addition, given the relevance which is acknowledged in the cohort fertility to the parity, the analysis will be carried out by birth order, in fact a further test on the capability of the methods to adapt to various fertility patterns.