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2021 Census, MYE Rebasing & Implications for Demographic Forecasting

What are Mid-Year Population Estimates (MYE)?

Between each Census, the size and structure of the population in the UK is estimated by the Office for National Statistics (ONS), taking into account the number of births and deaths each year and estimating the levels of internal (domestic) and international (overseas) migration.

The population is estimated at mid-year (30th June), hence the term ‘mid-year population estimate’. The drivers of population growth (births, deaths (natural change) and migration) are referred to as ‘components of change’.

Starting with a Census estimate, the population is ‘rolled-on’ to the next year, by adding the births, subtracting the deaths, and estimating the levels of in- and out-migration. This happens each year until another Census occurs.

What happens following a Census?

Following a Census, ONS reviews the previous intercensal MYEs (e.g. between the 2011 and 2021 Censuses). As ONS states, “Mid-year population estimates … immediately following a census are considered to be highly reliable”. With increased distance from a Census, the reliability of the MYEs is reduced.

The post-Census review involves a ‘reconciliation and rebasing process’ to better align one Census estimate with another and identify the possible reasons for ‘intercensal drift’.

ONS says: “rebasing follows on from the reconciliation process and aims to spread the differences observed out across the last decade in a plausible fashion (to reflect years of the decade when a component of change was contributing to that difference), apportioned to the most likely component of change.”

This review process is currently ongoing following the 2021 Census. In February 2023, ONS published its reconciliation of the MYEs with Census 2021. This analysis compared the MYEs for local authorities that would have been generated by rolling on the 2020 MYE, against those rolled on from the 2021 Census. At the local authority level, the rolled-forward MYEs were more likely to overestimate males than females compared with the census-based MYEs.

ONS is planning to publish a series of rebased MYEs for each local authority district in England and Wales in spring 2023 (April/May).

How different are the Census estimates & MYEs?

The Census 2021 estimate of population for England and Wales was slightly smaller than the mid-2020 population estimate. It is likely therefore, that following the reconciliation and rebasing process, “there will be small revisions back across the decade and England and Wales, to ensure a credible time series” (ONS). At a national level, ONS identifies international migration as the most likely cause of the differences seen between the MYEs and the Census estimates.

Whilst the difference between the 2020 MYE and the 2021 Census count is relatively small at a national level (-0.2% difference), in some local authority districts, the scale of change between 2020 and 2021 is substantial…

In Camden, the 2021 Census population estimate is nearly 70,000 lower than the 2020 MYE (-25%). In Cambridge, the 2021 Census population estimate is 21,000 higher than the 2020 MYE (+17%). Whilst it could be the case that these differences reflect reality, it is more likely the case that the populations of Camden and Cambridge have been respectively over and under-estimated between the 2011 and 2021 Censuses.

ONS is also planning to produce “a ‘Census Day minus one year’ estimate, to … better understand how much of the difference between census estimates and mid-year estimates occurred in the year leading up to census, which was during the coronavirus (COVID-19) pandemic.”

Why is this update important?

The MYEs provide a crucial input to population forecasts. Edge uses the MYEs to configure future population growth scenarios, drawing migration assumptions from the historical data. For example, a ‘5-year trend’ scenario would use the latest five years of MYE migration data to derive future rates of migration for a given area. The latest MYE also provides the ‘base’ population to the forecasts, defining the starting size and structure of the population.

In many areas, the scale of change between the 2020 MYE and the 2021 Census has resulted in substantial alterations to the population, not just in terms of the overall size, but also the population age/sex profile.

The rebased MYEs are therefore an invaluable update to our population forecasting models (POPGROUP and VICUS).

For more information on the Edge forecasting methodology or our use of Census data, get in touch with the team at

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