The Cohort-Survival Method

This is the standard method of population projection used by official (government) agencies in most advanced countries. It is not a rigid method, and can be adapted in a great variety of ways to suit the data available or the needs of the analyst whilst at the same time retaining its underlying logic.

The general form of the cohort-survival method is as follows. Males and females by single-year age groups are tabulated separately, the figures being extracted from the latest available census. Next the net migratory change for the first year is allowed for be the addition (or subtraction) of the assumed change for each age-group of males and females. Then the appropriate age-specific birth rates are applied successively to each group of women in the child-bearing range (usually 15-49 last birthday); the resultant births are divided into males and females, adjusted for mortality in the first year and entered in the next column, first row of the male female tables. Finally, age specific mortality rates of or survival rates are applied to each age-group of males and females to estimate the numbers who will survive to the next year (i.e. of their life and of the projection).
This sequence is repeated until the projection data is reached.

A very common simplification is to work with five-year (quinary) age groups 0-4, 5-9, 10-14,…etc. and to project by quinquennial periods, whilst possibly reducing the accuracy of the method and the degree of control which can be exercised over the process, the gains in time and the reduction of tedium (if manual calculation is used) may be felt o outweigh the losses. Slightly different data are needed e.g. five year fertility ratios to estimate births and, of course, the pattern of migration assumed must be expressed by five-year periods.