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Change the past…change the future…

The importance of the rebased Mid-Year Population Estimates (MYEs) for population forecasting

Dr Kate Staines

Head of Housing Analytics, Edge Analytics

What are MYEs?

The 2021 Census has provided an important update to our understanding of population characteristics at a local, regional and national level. It is, however, a snapshot in time. Between each Census, population estimation is therefore necessary.

The Office for National Statistics (ONS) estimates the size and structure of the population at mid-year (June 30th), taking account of recorded births and deaths, and estimates of domestic (internal) and international (overseas) migration.

These mid-year population estimates (MYE) are produced each year until the next Census takes place. With ten years between each Census, an important issue is raised: the longer it has been since the last Census, the less robust the estimates become. For some local authorities, the population growth trend implied by the intercensal MYEs is considerably out of step with the 2021 Census population estimate, suggesting some element of population mis-estimation has occurred since the previous Census.

Following a Census, the ONS revisits the intercensal MYEs, producing a ‘rebased’ time series to better align the estimates between Censuses.[1] The rebased MYE time series for England and Wales is due for release at the end of September 2023 and will provide a vital update to our understanding of population change at a local-authority level over the last decade.

London: Difference between 2020 MYE and 2021 Census Population

MYE 2020 and Census 2021 population comparison.

Why is the MYE rebasing important?

Edge Analytics’ population forecasting models are configured with a range of growth assumptions but at the core of each model are assumptions relating to births and deaths (fertility and mortality), internal (domestic) migration, and international (overseas) migration. Referred to as the ‘components of change’, these assumptions drive population growth. The forecasting models can also be configured to consider the impact of planned housing growth, and/or employment growth.

In our forecasts, the fertility, mortality and migration assumptions are typically derived from historical trends observed over a defined time period, drawn from the MYEs. This approach is aligned with that used by the statistical agencies, which use this historical evidence to produce a range of official trend projections for the UK countries and constituent local authorities.

The MYE rebasing is important, as it not only provides us with an updated picture of population growth across England and Wales over the last decade, but it also enables us to derive updated fertility, mortality and migration assumptions for use within our demographic forecasting.

‘The release of the rebased MYEs from ONS represents a critical piece of the complex demographic jigsaw in that it provides a crucial link between the most recent Censuses. The rebasing tells us, for local areas, how the current picture was arrived at and what were the key drivers of growth, decline, or change over-time? Understanding these historical patterns will help shape future investment and policy decisions across both business and local government.’

Richard Culf, Chief Data Scientist, Edge Analytics

How is this evidence used by our clients?

Our demographic profiles and forecasts are used by a wide range of clients to inform service, resource, and infrastructure planning over a range of timescales and at a variety of geographies. The MYE rebasing provides an opportunity to reformulate the assumptions underpinning our demographic forecasts, the revisions to which could have important implications for business planning. The core requirement for any industry is that they receive the most dependable, current, and robust evidence available. In all our forecasting, we consider a variety of data inputs and assumptions, providing a range of growth outcomes for consideration.

For the water industry, we use our unique macro-micro forecasting model, VICUS, to generate population and property forecasts that are used to inform the development of water and wastewater resource management plans. Regulatory guidelines set by Ofwat and the Environment Agency stipulate that resource management plans should be informed by the latest demographic evidence and housing growth plans of the local authorities covered by the relevant water company. We use our VICUS forecasting technology to produce ‘housing-led’ forecasts, evaluating the demographic implications of planned housing growth at a micro-scale. Adjustments to the underpinning fertility, mortality and migration assumptions as a consequence of the MYE rebasing are likely to alter the resulting growth outcomes from such scenarios.

For local authorities, we use POPGROUP forecasting technology to generate demographic forecasts to inform the Local Plan development process across England and Wales, assisting in the assessment of local housing and employment needs. For many local authorities, the 2021 Census introduced an element of uncertainty around the intercensal profile of population growth. For example, in many London authorities, the 2021 Census population was lower than anticipated (see map above), suggesting that the MYEs had over-estimated the scale of growth between 2011 and 2021. Where population forecasts have been developed using this previous MYE evidence, and Local Plan policies informed by this, it is of vital importance to consider the rebased MYEs and the potential implications of these for future plans and policies.

How can we help?

With frequent data updates and periodic data revisions, identifying and understanding the demographic statistics and trends that are relevant to your business can be challenging. Our experienced team of analysts and demographic specialists can help you navigate this ever-changing landscape by producing robust and transparent analysis and insights to help you understand your market and the drivers of changing demand, both historically and in the future.

Edge Analytics’ range of proprietary tools, demographic datasets, and forecasting models provide robust population and housing forecasts for resource, service and business planning. Trusted by public and private sector organisations across the UK, Edge provides defensible, dependable insight that has stood up to regulatory scrutiny again and again.

To discuss how Edge Analytics can support your next project, email us at

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