Building the richest picture of our population

A group of people next to the River Thames

Population statistics underpin a vast array of analysis, from unemployment rates to health outcomes, and are vital to decisions about all public services. Pete Benton explains how, this year, with new census outputs and more frequent statistics from new data sources, the ONS will deliver richer and more timely insights than ever before.   

It is going to be an exciting year, with broadly three groups of population statistics to look forward to. We’ll have:  

  • Results from Census 2021, giving a picture of the population in March 2021. 
  • 2021-based population and migration statistics, rolled on from the census results. 
  • New experimental statistics about the population as it is in 2022. 

The 2021 Census Results 

The ONS delivered a very successful census operation – with a return rate over 97% – and from this we will be able to provide a fantastic picture of our population. We aim to publish the first results in late May and will confirm the exact date once we’ve completed our processing and quality assurance [please see this statement published on March 1 for our latest position]. We are working in partnership with local authorities and other experts, bringing in a variety of other data sources during the quality assurance of the 2021 Census results, to have the highest confidence in the figures we publish. 

The beauty of the census is the richness of the picture it paints of the nation. Releases covering the full detail from the census will follow through to Spring 2023 giving an ever-richer snapshot of the population as it was in March 2021, faster than any previous census.  We’ve already started to use Census 2021 data to inform management of the COVID-19 pandemic.  

An important part of the picture will be a small number of census topics that give insights into life during pandemic – statistics on commuting patterns, in particular, are likely to show significant numbers of us working from home.  Reporting on these will be an important and fascinating part of the published outputs.  We will monitor ongoing change since census day using new sources of data, such as using aggregate mobile phone data, to understand changes in patterns of travel to work. 

2021-based Population and migration statistics, rolled on from the Census 

Whilst the census gives us a brilliant snapshot of society once a decade, we need to continue to understand how the population changes over time. We will initially do this by publishing our official mid-year population estimates in September. These will relate to the population as at 30 June 2021 and will show how the population has changed nationally and locally since Census Day, as well as adding to the long time series we have of the population and how it’s changed over the decades.  

We will also continue to improve our international migration statistics using a range of new data sources and methods.  Our latest estimates will be published in March 2022, relating to June 2021. We continue to develop these experimental statistics, bringing in new sources of data into our models, such as data about migrants from the Department for Work and Pensions, as well as reviewing the definitions of migration to ensure they meet the needs of users.  

We’re currently considering whether some of our existing international migration outputs, such as UK population by country of birth and nationality stock estimates from the Annual Population Survey (currently planned for May), add value or whether the census estimates, due out shortly afterwards, provide users with the best information. We will announce the outcome of that decision as soon as possible.  

New, experimental statistics about the population as it is during 2022 

Going forward, we want to provide population and migration statistics more quickly and more frequently.  I have previously described how we intend to transform our population and migration statistics using administrative data sources, and throughout this year we will continue to publish research updates, building towards “experimental” monthly age/sex profiles of the population relating to 2022. 

This will start with a proof of concept for admin-based monthly population estimates as soon as possible after the first Census 2021 results are released, using information from health, tax, benefits and education data sources, among others. Providing these monthly population totals in the future will allow decision makers to have the most up to date information possible on the dynamics of the population and ensure that public services can be provided to the right population groups in the right places. This has never been more important than now, as we emerge from the pandemic, and since our departure from the EU.  

What will the new world of population and migration statistics look like? 

We will significantly enhance our understanding of our new data sources and methods by comparing the results of our research with the 2021 Census results, and by inviting feedback from users as we have done previously.   

As our methods mature, we will embed them into our official estimates and move on from the “experimental” status. 

In future, users are likely to see more timely “provisional” population and migration estimates as close to the reference date as possible, based on the best data available at the time, followed by “final” estimates as more data become available.  This is similar to our approach for National Accounts which describe our economy.  We’ll need to work with users to understand the optimal trade-off between more timely provisional estimates and increased accuracy as more data become available. 

Exactly how this progresses, and how soon we can publish the first 2022-based experimental statistics, will depend on many factors including what we learn about the source data as we compare with the 2021 census results; the extent to which we need to modify our methods in light of what we find; and the feedback from users about what is most beneficial to them.  

What we can say for now though, is that we’re moving towards a population and migration statistics system that will deliver richer, more detailed and more timely updates to census information than we’ve ever had before, with the breadth and detail continuing to grow over the coming years. This will help us to provide the best picture of the UK’s population in the long term. It also supports the latest work to meet our commitments following the recommendations in the review of population estimates and projections carried out by the Office for Statistics Regulation in 2021, an update article on which we have also published today. 

For more details about our planned outputs this year, please see this published table. We’ll keep this updated as our research develops, and I’ll say more in another blog before we publish the first census results. 

Image of Pete Benton, Deputy National Statistician for Health, Population and Methods

Pete Benton is Deputy National Statistician for Health, Population and Methods