## Cohort-component population projections

### Introduction

Cohort-component method population projections:

- model the age-sex structure of populations and not just their size
- model the

The procedure for making cohort-component population projections was developed by Whelpton in the 1930s. It can be thought of as an elaboration of the ideas encapsulated in the demographic balancing equation:

*P(t+n)*= *P(t)* + *B(t)* − *D(t)* + *I(t)* − *E(t) *

where:

*P(t)*is the population at time*t**B(t)*and*D(t)*are number of births and deaths occurring between*t*and*t*+*n*.*I(t)*and*E(t)*are the number of immigrants and of emigrants from the country during the period*t*to*t+n*.

This equation reminds us that there are only two possible ways of joining a population: one can be born into it or one can migrate into it. Similarly, the only ways to leave a population are to emigrate or to die. At the global level, nobody has joined the human population by immigrating and only a few unfortunate astronauts have emigrated and not returned.

Cohort-component projections extend this concept to individual age cohorts. They make use of the fact that every year of time that passes, every member of a population becomes a year older. Thus, after 5 years the survivors of the cohort aged 0-4 years at some baseline date will be aged 5-9 years, 5 years after that they will aged 10-14 years, and so on.