Meeting the challenges in population estimation

People walking across a footpath

The coronavirus pandemic has made measuring the size and changing nature of the UK population more challenging, particularly with many of our face-to-face surveys suspended. Here Rich Pereira explains how we are adapting to these challenges by introducing new sources and methods to produce the best possible population estimates. 

Understanding how the population is changing is a crucial part of our work at the ONS. Our statistics inform decision-makers in central and local government as well as providing the public with a view of the society we live in.

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.

Mid-year population estimates

In June 2021, we will publish our next official population estimates for mid-year 2020, which will use additional data and provide more detailed statistics on the size and structure of the population by age, sex and local area.

To understand how the population is changing, we need to measure the components of births, deaths, and migration (both international and internal to the UK). The pandemic has affected some of the data sources we use to measure these, as outlined in our previous blog on early indicators of the UK population. For example, birth registrations were initially delayed, but we now have sufficient data coverage for our mid-2020 estimates.

For international migration, the usual source of migration estimates – the International Passenger Survey – was disrupted by the pandemic and so we have made innovative use of alternative data sources to produce modelled estimates for the period between March and June 2020. These will feed into our population estimates and will help pave the way for incorporating transformed migration statistics based on further administrative data in future.

Alongside this, we need to consider the way lockdown restrictions impacted different groups, such as where university students were living during the pandemic and how to reflect this in our statistics. In addition to our usual internal migration estimates, we therefore plan to produce alternative data that shows how different the size of local populations would be if students had been able to remain at their term-time address. Together, these will help planners and decision-makers more fully understand the impact of the pandemic on the population.


Last weekthe Office for Statistics Regulation published a review of population estimates and projections.

We welcome the report’s recognition that we use internationally recognised methods and sources as the basis for population estimates and projections that are fit for purpose for national level numbers.  However, we recognise the need to keep these current and responsive especially at some lower levels where there is more variability.

We are constantly looking to develop the way we work with users of our statistics and welcome the findings in the report relating to engagement, particularly at a local level and with central departments to promote understanding of uncertainty and the appropriate use of statistics.

We are already progressing much work in this area. We continue to build on new methods, for example exploring the ways in which students leaving university are counted, and the established population and migration statistics transformation programme is making increased use of administrative data.

Census 2021, which has been a great success with 97% of households responding so far, will also provide a solid foundation for estimating the size of the population going forward. The initial results from this will be out next spring.

Our plans for reweighting the Labour Force Survey

Today, we have also published an article setting out more detail on the methods we have developed to improve the population figures used in the Labour Force Survey (LFS) to calculate some of our employment figures, to ensure they better reflect how the UK population has changed during the pandemic. This builds on the article we published in March using HMRC ‘Real Time Information’ data to explore how payroll employment by nationality changed through the pandemic, what this tells us about the wider population and how we can use these insights to reweight the LFS.

It’s important to note that this is different to the methods we use to produce mid-year population estimates and projections. Our approach is to use the best possible data and sources for each purpose. RTI data provides the best source for reweighting because this is more comparable with LFS employment data and provides timely information to the end of 2020 by country and regions of the UK. We plan to implement the reweighting in our labour market statistics in July 2021. We will continue to review our methods and carry out any further adjustments to the weighting as needed as further timely and relevant data sources on the population become available.

In August we will be doing the same with the Annual Population Survey, which is used to inform our non-UK population stock estimates. We will then subsequently publish our population of the UK by country of birth and nationality output.


We are on track to deliver both our mid-year population estimates and LFS reweighting, although we are taking different approaches with both at this time, for reasons outlined.

Across our work on labour market, population and migration statistics, we will continue to work closely with our colleagues across the Government Statistical Service to bring together all available data and deliver the best possible insights and will keep users informed on our plans.

Our work across these areas will continue to evolve as we bring in new data sources and develop our methods and approaches. We will continue to provide regular updates and are always keen to receive feedback  please contact us at with any comments.

Rich Pereira is the Deputy Director, Centre for Ageing and Demography