Helping ONS improve population estimation

CPC researchers have been informing the new methodology used by the Office for National Statistics (ONS), by helping them to adapt to the challenges of measuring changes in the UK population during the Covid-19 pandemic. Understanding how the population is changing is a crucial part of the work of ONS. Their statistics inform decision-makers in the central and local government, and provide the public with an overview of our population.

Members of CPC’s Modelling strand, including Professors Peter W F Smith and Jakub Bijak, Dr Erengul Dodd and Dr Jason Hilton, have been meeting with ONS to advise on measuring uncertainty in population and migration estimates, and on assessing the impact of shocks, for example the Covid-19 pandemic, on mortality forecasts. The study team have also met with the ONS and Public Health England (PHE) to discuss their work using the ONS weekly mortality statistics to estimate excess mortality due to Covid-19. They have been comparing current PHE methods with a novel, dedicated method of estimation developed at the CPC.

Alongside this, the CPC team have been working on methodology to estimate migration without an International Passenger Survey (IPS). The IPS collects information about passengers entering and leaving the UK, and has been running continuously since 1961, but was suspended between March 2020 and January 2021 due to the coronavirus pandemic. The CPC study team have been investigating short-, mid- and long-term solutions to estimating UK migration flows based on administrative data. These estimates will ultimately feed into official UK population estimates, and will help transform migration statistics based on administrative data in the future.

Professors Smith and Bijak and Drs Dodd and Hilton, along with CPC associate, Professor Jon Forster, also worked with the ONS to develop state-space models to estimate UK international migration and to make innovative use of the available data sources and methods. The methods and findings were published in the ONS report ‘Using statistical modelling to estimate UK international migration’, with the provisional modelled estimates suggesting that the total net migration was negative from April to June 2020, with more people leaving the UK than arriving. Still, the findings also emphasised large uncertainty around these numbers and trends, since it is not possible to accurately quantify international migration during that unprecedented period.

Richard Pereira, Head of the ONS Centre for Ageing and Demography, commented: “In April, we published early indicators of the UK population, which provided important insights on the size and age structure of the UK using the latest available data. These provided an initial view of how the population was changing in the year to mid-2020, which includes the first part of the pandemic. Counter to some external expectations that the population fell, these estimates showed the UK population grew – by around 0.5% – although this did mark one of the smallest increases seen on record. In the official population estimates released on 25 June 2021, we estimated that the UK population in the year to June 2020 was 67.1m, an increase of 0.4% over the past year which is the slowest growth since 2001. The effect of the pandemic on mortality can largely explain the slowing of the growth rate between 2019 and 2020.”

The latest official ONS population estimates for mid-year 2020, providing more detailed statistics on the size and structure of the population by age, sex and local area, are now available on the ONS website.

Further reading

Meeting the challenges in population estimation (National Statistical, ONS blog)

Population estimates for the UK, England and Wales, Scotland and Northern Ireland: mid-2020 (ONS Population Estimates)

How many people live in the UK? (National Statistical, ONS blog)

Using statistical modelling to estimate UK international migration (ONS Working Paper)

Early indicators of UK population size and age structure: 2020 (ONS Population Estimates)