Population estimates and coverage denominators

How do statistics offices estimate the population?

The Office for National Statistics published the mid-2020 population estimate for the United Kingdom. How do statistics offices estimate the population?

The cohort component method

The main way of estimating a population is a cohort component method. This uses data sources to inform components of demographic change. Ten-yearly censuses ‘base’ population estimates, to roll forward.

The three major parts are:

• Natural change: Population estimates for the previous year have one year added on their age. Analysts add births in the 12-month period. They deduct deaths by age, sex, and usual area of housing.
• Migration: People move into a country (immigration), but others leave (emigration). There is also internal migration within the country. There is no mandatory or comprehensive annual population register in the UK. That makes migration statistics the hardest to estimate.
• Special populations: There are some populations which sit outside by usual migration estimates. For the UK, that means prisoners and members of the armed forces.

The method estimates natural changes, net migration, and changes in special populations. The ONS produces population estimates for England and Wales. National Records Scotland and Northern Ireland Statistics and Research Agency…