The UK is changing the way it measures migration.  

The current migration figures are estimates based on the International Passenger Survey (IPS), which is run by the Office for National Statistics (ONS). The ONS’s plan is to replace this with a new series based on administrative data. This process has been accelerated as the IPS is suspended due to the spread of Covid-19.  
This Insight explains how migration statistics are transforming and the implications. The ONS has a related programme to transform its population statistics, which will not be covered here. 

The status quo 

Since 1964, the UK’s main estimates of international migration have been made using the IPS, which is a survey of people arriving and departing at UK airports and ports.  

The IPS was designed mainly to produce information about tourism not long-term migration. Out of around 800,000 people who are interviewed per year, only around 3,000 say they are migrating to or from the UK. The information given by these people is then used to estimate migration flows on a national scale.  

Over the years, the ONS has supplemented the survey findings in various ways with other data. For example, asylum seekers would not be picked up in the IPS sample so they are added onto the IPS estimates. This plugs a gap in the survey’s coverage and improves the accuracy of the overall migration estimates. 

Why do migration statistics need to change? 

Despite these efforts, the survey has often produced estimates which differ from other sources of information.  

When the results of the 2011 Census were analysed, it was found that the UK’s population was 464,000 higher than previously estimated. 

This was the fault of the IPS undercounting immigration, which is part of population change. The survey had missed the growing number of people coming to the UK via its regional airports from countries that joined the EU in 2004. This was because its sampling method had not been adapted to reflect changing migration patterns. 

The IPS has also been found to underestimate the number of migrants who leave the UK. In particular, it underestimates the number of students who leave the UK at the end of their studies. This was revealed when the Home Office began collecting exit checks data in 2015.  

As the chart below shows, the IPS underestimated non-EU emigration by around half in 2016-17. It also shows different ‘reasons’ for migration. People might have multiple reasons for migrating and the reason they give in response to a survey might differ from the category of visa they hold. 

A chart showing that Home Office visa data show higher levels of emigration than what was estimated by the International Passenger Survey.

Why is the transformation happening now? 

In 2013, the Public Administration Select Committee (PASC) concluded from its inquiry into migration statistics that they were “blunt instruments for measuring, managing, and understanding migration to and from the UK.” The PASC’s Chair Bernard Jenkin MP described them as “not fit for purpose in the longer term.” 

For a long time, the IPS was the best means of estimating migration. There are now several administrative datasets that give us more accurate, if only partial, data on migration. A summary of the available sources can be found at the end of this article. 

By 2017, the alternative data sources were in a promising-enough state for the ONS to announce it was starting work in earnest to bring about a “new era for migration statistics.” 

What has the transformation programme done so far? 

The ONS has already begun using aggregated data from the Home Office and the Department for Work and Pensions (DWP) to improve the accuracy of the estimates coming from the IPS.  

The first major adjustments to the IPS-based estimates came in August 2019: 

  1. The non-EU net migration figures between 2013 and 2019 were reduced to account for the IPS having underestimated the emigration of former students (as illustrated above). 
  1. The EU migration figures were increased between 2011 and 2016 after analysis of National Insurance Number registrations showed that immigration from Eastern Europe (EU8 countries) had been around twice as high as estimated by the IPS. 

The result of these changes is shown in the series of charts below. 

The original migration figures, which were based mainly on the International Passenger Survey, showed that non-EU migration was higher in each of the last 10 years.

A graph showing Original net migration estimates, by nationality

The revised figures show EU migration having been higher for around 4 of those years (2013-2017).
The original figures were adjusted based on analysis of Home Office visa data and DWP/HMRC employment, tax, and benefits data.

A graph to show adjusted net migration estimates, by nationality

But there was already uncertainty in the original estimates.
This is not to say that the revised estimates have no uncertainty – they still do, it just cannot be measured in the same way.

At the same time, these migration statistics were re-designated from ‘national statistics’ to ‘experimental statistics’. Experimental statistics are not expected to have completed stringent checks for robustness and accuracy. The ONS itself suggested the change to be able to make changes to the series more freely. 

The plan was for the ONS to continue exploring the available administrative data, in combination with the IPS and gradually move away from using the IPS at all. 

The impact of Covid-19 

On 16 March 2020, the IPS was suspended for safety reasons due to Covid-19. Given that it takes some time to process the IPS data, this does not immediately affect the production of migration statistics.  

It will become a problem from November of 2020, when this gap in the data means the usual migration estimates cannot be produced. 

On 21 May 2020, the ONS announced it would be letting the pandemic force its hand in this respect, by moving to the next phase of the migration statistics transformation in November.  

Next steps in the migration statistics transformation


International Passenger Survey (IPS) suspended due to covid-19.
Migration Statistics Quarterly Report (MSQR) for the year ending December 2019 released, with estimates still based mainly on the IPS.
Final MSQR that uses the IPS (at least for the time being) due to be released. This will cover the year ending March 2020.
First MSQR that does not use the IPS but with estimates based on administrative data from the Home Office, DWP, HMRC, and possibly other sources. This will be for the year ending June 2020.

More detailed migration data for the year ending December 2019 (which does still use the IPS), will also be released as planned.


Free movement of EU nationals to the UK ends. Current Government policy is that as of 1 January 2021, all foreign nationals who do not have permanent residence/ indefinite leave to remain in the UK will require a visa.*
MSQR release for the year ending September 2020. This will also be based on administrative data, as opposed to the IPS.
The UK Census is due to take place, with the results to be released from 2022 onwards.

*There are, and will continue to be, some very limited exceptions to this.


The end of free movement  

The ending of free movement means that, as of 1 January 2021, EU/EEA nationals will become subject to the UK immigration system. This will increase the proportion of migrants covered in the visa data, whereas currently there is limited administrative data on this population. 

The ONS has committed to providing regular updates on the progress of the transformation programme and invites feedback from users on its proposals. 

Further reading 

Population and migration statistics system transformation – overviewONS

Migration statistics: Where do they come from? – A detailed explanation of the IPS, House of Commons Library 

Download the data sources and full timeline

Migration Data Sources – HoC Library June 2020

Migration Statistics Transformation Timeline – HoC Library June 2020

About the author: Georgina Sturge is a researcher at the House of Commons Library specialising in social and general statistics. 

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