Relationship between the Denominator Age of the Child-Woman Ratio and the Accuracy of Regional Population Projections

<p>The child–woman ratio (CWR) is frequently used as a fertility parameter in projecting the regional population and is commonly calculated as the value obtained by dividing the number of children aged 0–4 years by the number of women aged 15–49 years. However, to improve the accuracy of population projections, the CWRs are suggested to be calculated using different denominator ages based on the following two ideas: the deviation in age-specific fertility rate and the strength of correlation between total fertility rate and CWR at the prefectural level. These ideas remain merely hypotheses because empirical analysis of them is lacking. This study aimed to verify the validity of previous hypotheses using the model population projections of all municipalities and propose a new analytical framework that expresses the logical relationship between the denominator age of the CWR and the accuracy of regional population projections.</p><p>By analyzing the projection error of the population aged 0–4 years using different denominator ages of the CWR through model population projections, these hypotheses are valid in the projection beginning with the 1980 Population Census but are invalid in that beginning with the 1990 or 2000 Population Census. The reason for this is thought to be that the influence of age structure changes was not taken into consideration. Therefore, for grasping the influence of age structure changes, projection errors of the population aged 0–4 years were decomposed to the following three projection errors: the age-specific CWR, age structure ratio, and the population of denominator age of women. This is the new analytical framework between the projection accuracy and the denominator age of CWR. The denominator age with high projection accuracy cannot be determined before projection even if this framework is used. However, the difference in projection errors by selecting the denominator age of the CWR will be reduced, since age structure changes become smaller as reflected by the decreased number of births in the past. In other words, the significance of selecting the denominator age diminishes for the future. Thus, this study recommends the use of the most common CWR with a denominator age of 15–49 years in regional population projections.</p>