cohortcomponent

The cohort-component method, also known as the cohort-component model, is a demographic projection technique that models population change by age and sex through separate components of change: births, deaths, and international migration. It follows cohorts as they age over time, applying survival to move them forward and incorporating new entrants in the youngest ages from births and migration.

Inputs for the method include the initial population by age and sex, age-specific fertility rates, mortality or survival rates, and migration by age and sex for each projection interval. These components can be estimated from data and may be held constant or varied across scenarios to explore different futures.

The projection process works as follows: for each time step, births are computed by multiplying the number of women of reproductive ages by the corresponding fertility rates and then assigning the births by sex. Deaths are applied by aging the population and applying survival rates, so each age-sex group moves to the next age class. Net migration is added by age and sex. After births, deaths, and migration are accounted for, the population is aged forward, with newborns entering the age-0 group and survivors advancing to the next age group; the procedure repeats for subsequent intervals.

Uses and limitations: the method is widely employed in national and subnational population projections, planning, and policy analysis due to its transparent accounting of drivers of change and its internal consistency. Limitations include data intensity, sensitivity to input assumptions, and potential oversimplification of subpopulation heterogeneity and feedback effects.