The subnational population estimates are based on:

1. The share of population at the first level administrative division from national population censuses and national estimates
   
2. Annual total population estimates from the World Development Indicators.
   

The subnational population estimation methodology features the following. The population data from national censuses and estimates by national statistical offices or the Center for International Earth Science Information Network (CIESIN) are adjusted for mid-year (July 1) by using constant population growth rate between two data points. For the years for which census or national estimates are not available, the population figures are interpolated or extrapolated. For the interpolated data, a constant growth rate of population between two data points is used to obtain first-level administrative division population estimates. For the extrapolated data, the average growth rate of the earliest (or latest) 3 years is used to calculate first-level administrative division population estimates. Then the percentage share of each first-level administrative division population to the sum of all first-level administrative division areas is calculated. For countries where there is only one data point, it is assumed that the percentage share of each first-level administrative division population to the total population remains constant for all the years. Finally, first-level administrative division population estimates are calculated by applying the percentage share of each first-level administrative division to the World Development Indicators' annual total population estimates.