Review of population estimates and projections produced by the Office for National Statistics

10 May 2021
Last updated:
11 May 2021


Scope of this review

Our review was initiated in response to concerns raised with us in November 2020 regarding the population projections and mid-year population estimates for Coventry. The concerns were around the perceived inaccuracies of the population estimates on which the household projections and subsequent housing need are based. Upon announcing our review, we received further concerns from a number of other areas with related concerns.

While we investigated the concerns raised with us as part of our review, we also considered whether the methods and approaches are as good as they can be, and to what extent the estimates and projections can bear the weight put on them where they are used in decision making, in line with the principles set out in the Code of Practice for Statistics. It is not within our remit to regulate operational decisions made by government or local authorities, nor to form a judgement on decisions about government policy.

Our review is based on national and subnational mid-year population estimates for England and the biennial national and subnational population projections for England.

Some of the concerns we received related to the way population estimates feed into the household projections from which housing need is determined. We took the decision to focus our review on population estimates and projections, as an underlying source for household projections, to understand in the first instance whether the concerns raised with us affect other areas or other policies beyond house building.

To inform our review, we carried out interviews and focus groups with individuals with an interest in or experience using population estimates and projections. This approach ensured that we obtained the views of a wide range of users from differing backgrounds. These meetings took place between February and March 2021. A list of users we engaged with is provided in Annex A.

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The statistics

The population estimates and projections are important data with implications for many other statistics as they are used for weighting or as a denominator. For example, labour market statistics are based on sample surveys that use the population estimates to be scaled up for the population. The projections feed into local planning decisions which can have a long-term commitment and therefore the impact of issues can have far reaching consequences.

The population estimates and projections for England and Wales, at national and subnational levels, are a long-standing set of data produced by ONS. The subnational mid-year population estimates for England and Wales are calculated first and the national estimates are produced by aggregating the subnational estimates. The estimates are produced annually in June and are initially rebased following a census year and then the component parts of births, deaths and migration are rolled forward each year by applying the cohort component methodology.

The population estimates then act as the starting population for producing the population projections. Projections for successive years are produced by taking the starting population for each age and then accounting for net migration, births and deaths for each onward year projected. Again, the subnational population projections take the mid-year population estimates as the starting point and for the principal projection, data for the preceding five years are used. The principal projection is the headline figure presented in the main statistical bulletin and analysis. ONS also produces several variant projections which are based on differing underlying assumptions to the principal projection. The projections based on these trends are then constrained to the totals used in the principal population projections for England.

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Wider Context

In March 2014, the National Statistician recommended that the census in 2021 should be predominantly online, making increased use of administrative data and surveys to both enhance the statistics from the 2021 Census and improve statistics between censuses. The government’s response to this recommendation was an ambition that censuses after 2021 will be conducted using other sources of data. In 2023, ONS plans to present its recommendations to government as to the future of census arrangements, in the light of the progress that has been made in switching to an Administrative Data Census.

As part of this ambition for an admin-based approach, the ONS has been working to transform its population and migration statistics more broadly. The current population system is heavily reliant on the decennial census which affects the quality of population estimates as we move further away from the census year. Using its data-sharing powers through the Digital Economy Act 2017, ONS has been progressing research into how it can bring a range of government data sources together to build an integrated system for measuring population and migration.

This report has been completed within this period of change and development for ONS, with the timing also coinciding with the Census in England and Wales. The 2021 Census will provide ONS with a refreshed foundation to estimate from and may lead to some estimates from the past decade being rebased. As such, our recommendations have been written with a forward look. ONS should consider our findings in respect of its plans for future migration and population statistics and consider if our findings around its approach to user engagement and feedback may have a wider impact across ONS as a whole.

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