Rolling forwards and rolling back

How do different population estimates arise between censuses?

Population estimates are useful statistics for many purposes. The needed granularity, such as by age group or local area, can differ between purposes.

How do analysts estimate a population between censuses?

A prominent form of estimation is the cohort component method. That starts with the ten-year census ‘base’, to roll forward in intervening years. The estimates uses many data sources, to inform parts of demographic change.

The three main components are:

• Natural changes: People age, so analysts add on one year to the prior population estimate. New births join populations, whilst deaths deduct.
• Migration: People enter into the country, and others emigrate. There is internal migration between places within the country. In absence of a population register, migration is the hardest component to estimate.
• Special populations: Some populations sit outside this typical method. For the United Kingdom, that means groups of prisoners and armed forces.

National Records Scotland details the cohort component method. (Image: NRS)

This cohort component method estimates natural change, migration, and change in special groups. The Office for National Statistics…