- 89.12% of local authorities had a difference in their population of less than positive or negative 4.99% between the official mid-2021 population estimates for England and Wales, based on Census 2021 data, and the rolled-forward estimates from mid-2020.
- At the local authority level, the rolled-forward mid-year estimates were more likely to overestimate males than females compared with the census-based mid-year estimates.
- Excluding City of London, Camden showed the largest percentage difference (24.97%) between the rolled-forward and 2021 Census-based mid-year estimates for all local authorities in England and Wales.
- Patterns of difference at the local level can be complex, and further research is needed to understand the impact of rebasing at lower geographic levels, as well as how various components have fed into the differences explored in this article.
This article sits alongside the England and Wales level reconciliation report published on 28 February 2023. It seeks to explore the differences between the official Census 2021-based mid-year estimates (based on Census 2021 and accounting for births, deaths and migration in the period to mid-year 2021) and the 2021 rolled-forward (based on the 2011 Census with a decade of mid-years estimates with births, deaths and migration incorporated) mid-year estimates at a lower geographical level. See Section 3: Population estimates used in this report for a more detailed description of the two estimates.
This article also provides part of the suite of evidence that can be referred to as part of the consultation on the future population and social statistics system in mid-2023.
The Office for National Statistics (ONS) produces annual estimates of the resident population as at 30 June each year. These mid-year population estimates (MYEs) tell us how many people live in England and Wales as a whole and in each local area, and include information on age-sex structure.
This article examines the differences between the estimates but does not seek to attribute those differences to any specific cause. Initial information on what has contributed to the differences at the England and Wales level can be found in the main reconciliation report, and information on sub-national causes, such as internal migration flows, will follow in the sub-national rebasing planned for May 2023.Back to table of contents
Throughout this article we will be referring to two main mid-year estimates (MYE) of population.
Census-based MYEs are the official mid-2021 population estimates, these are based on the 2021 Census for England and Wales. The usual resident population as at Census Day (21 March 2021), by single year of age, is aged on to 30 June 2021 and then births, deaths and migration are accounted for.
Censuses provide the most accurate estimate of the population and therefore the reliability of MYEs is very high immediately following a census. Quality information on Census 2021 can be found in our Quality and methodology information (QMI) for Census 2021.
The 2021 rolled-forward MYEs use the population estimate from the previous reference date (2020 in this instance) as the starting point for estimating the population at the current reference date. The previous population estimate is aged on and data on births, deaths and migration are used to reflect population change during the reference period.
The census has evolved throughout the decades, providing an insight every 10 years into who we are and how we live. While the census and MYEs based on the census provide the best picture of society at a moment in time, how we produce population and social statistics is changing.
We are using a variety of data sources to provide more frequent, relevant and timely statistics. This will allow us to understand population change in local areas this year and beyond.Back to table of contents
The method used in years in which a census take place. The MYEs are based on the census estimates rolled forward only by the time between the Census Day and 30 June.
Components of change
Factors contributing to population change, including births and deaths (commonly referred to as natural change), and net migration. Migration includes movements of people between England and the various countries of the world (international migration) and between local authority areas within the UK (internal migration).
Moves made between local authorities, regions or countries within the UK. Unlike international migration, there is no internationally agreed definition.
The practice of using the population estimate from the previous reference date as the starting point for estimating the population at the current reference date. The previous population estimate is aged on and data on births, deaths and migration are used to reflect population change during the reference period.
Usually resident population
The standard United Nations definition is used, including only people who reside in a country for 12 months or more, making them usually resident in that country. As such, visitors and short-term migrants are excluded.Back to table of contents
The next stage is the production and publication of a rebased mid-year population estimates (MYE) series for England and Wales. Our goal is to publish this in April 2023. This will include the new official estimates of population for England and Wales for the years 2011 to 2020, to accompany the published census-based figures. These estimates will incorporate new migration estimates using our latest developments, where appropriate and available. This will allow us to spread any migration-attributed error over time, as well as make adjustments to other population groups (such as older ages and children).
Our aim is to follow the national rebasing with a rebased subnational MYE series in late May 2023. This will distribute the revised national-level estimates down to local authorities (LAs) and will incorporate the improved migration estimates and other improvements from the national rebasing. It will also include subnational specific improvements such as the improved internal migration methods (HELM) described in our Population estimates for the UK, mid-2021: methods guide, for the whole decade. We plan to provide tools to allow users to explore the impact of the rebasing for their local area, to allow them to understand how the rebased estimates differ from previous estimates, and the groups that are most impacted.
Alongside this, we have ambitious plans to further develop our Dynamic Population Model (DPM). The DPM uses a range of innovative sources to measure population counts and components of population change in a timely and coherent manner. These are not yet official statistics, while we develop our methodology and assess the quality of our outputs. We have also published our first attempt at producing DPM estimates for all 331 local authorities in England and Wales. In the summer of 2023, we will be updating this with improved data sources to compare against the Office for National Statistics (ONS) MYE for 2022.Back to table of contents
Office for National Statistics (ONS), released 2 March 2023, ONS website, article, Reconciliation of mid-year population estimates with Census 2021 at local authority level
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