Strategies for projecting populations
Two contrasting approaches exist for carrying out projections:
Total methods
Total methods calculate trends in the size of the population as a whole using a mathematical model of population growth. They may then distribute this total into sub-groups in ratio to the current structure of the population or an extrapolated forecast of its structure. Therefore, such approaches are sometimes known instead as ratio methods of projection.
Cohort component methods
Cohort component methods project each age group, sex and other category of interest separately. They then aggregate the results to obtain the total population. The term cohort emphasizes that an age group is made up of people born at the same time who go through life together. The size of a cohort at one age (and date) is strongly predictive of its size at other ages (and dates).
Many population projections combine both approaches, although projections dominated largely by the cohort component approach are by far the most common. Nevertheless, cohort component methods require many more input data and assumptions than total methods and may be inappropriate:
- if only estimates of total numbers are needed
- if the information required as input to component methods is lacking
- for very long-term projections.