Japan’s official subnational population projections accuracy: comparative analysis of projections in Japan, English-speaking countries and the EU

Masakazu Yamauchi, National Institute of Population and Social Security Research, Japan
Shiro Koike, National Institute of Population and Social Security Research, Japan
Kenji Kamata, National Institute of Population and Social Security Research, Japan

With the country’s total population having declined since its peak in 2008, future subnational population trends are among the major public concerns in Japan. The authors have been involved in official subnational population projections for Japan conducted at the National Institute of Population and Social Security Research (IPSS). This paper examines the accuracy of these subnational population projections. For the prefecture-level projections, the median absolute percent error (MedAPE) increases with the target duration of projections. MedAPEs for 5-year projections lie around 1%, while MedAPEs for 10-year projections around 2%. For the municipality-level projections, the percent errors increase to around 1.5% for 5-year and 3.5% for 10-year projections, respectively. Municipalities with small populations tend to have larger percent errors than those with large populations. In addition, we compared accuracy of IPSS subnational population projections with that of official subnational projections conducted by official agencies in English-speaking countries and by the EU. Using several measures, we found that the results of IPSS subnational population projections are more accurate than those conducted by official agencies abroad, although the IPSS projection model uses the cohort component method based on net migration rates, which is less sophisticated than the multiregional projection models used by other official agencies. The accuracy of the IPSS projections can be attributed to the relative stability of population changes in a country with an aging population and a lower level of international migration and number of foreign residents.