## Cohort-component population projections

### Introduction

Cohort-component method population projections:

• model the age-sex structure of populations and not just their size
• model the components of demographic change - fertility, mortality, and migration – and not just population growth.

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.