Introduction

Purpose of the review

1.1 This review looks at how the Office for National Statistics’ (ONS) is transforming its international migration estimates (referred to as ‘migration statistics’ in this report). It includes ONS’s plans to estimate long-term international migration (LTIM), which have been designated as experimental statistics since August 2019.

1.2 High quality migration estimates are crucial in helping the public and policy makers understand UK society. Migration statistics directly inform understanding of migration trends and characteristics. They are also a key component of population estimates in the UK. This means they have an impact on many outputs, including as denominators, such as for the unemployment rate and vaccination uptake, as well as to calculate survey weights to ensure surveys represent the full population, for example in the Labour Force Survey.

1.3 Our review of migration statistics will consist of a series of compliance checks. This report is the first of these, with the next one planned for later in 2022. As regulators, it is our role to support confidence in statistics by making sure that statistics serve the public good. Our aim through this review is for the transformed migration statistics to be trusted, high quality and of value in line with the Code of Practice for Statistics.

The statistics

1.4 International migration statistics are published by ONS, the Home Office and the Department for Work and Pensions (DWP). For example, DWP publishes statistics on National Insurance Number (NINo) allocations to overseas nationals entering the UK and the Home Office publishes quarterly immigration statistics.

1.5 Long-term international migrants (LTIM) are defined by the UN as those who change their country of residence for a period of 12 months or more. ONS previously published LTIM statistics quarterly in the Migration Statistics Quarterly Report (MSQR). The statistics were published as provisional rolling year statistics for the years ending in March, June, September, and December, with final, revised, figures for the previous calendar year published each November. Statistics on migration stocks are available in the Population of the UK by country of birth and nationality publication.

1.6 LTIM statistics were primarily based on data from the International Passenger Survey (IPS). The data were adjusted to allow for flows to and from Northern Ireland (with data provided by the Northern Ireland Statistics and Research Agency), those who change their migration intentions (for example, those who state an intention to migrate who then decide to return to their origin country within 12 months), and asylum seekers (with data provided by the Home Office). The IPS was a face-to-face sample survey of passengers entering or leaving the UK with a boost for migrant contacts. The IPS was carried out by ONS and was initially designed to produce data for National Accounts on tourism, but incorporated additional questions to measure migration using agreed UN definitions of migration.

1.7 In August 2019, Iain Bell Deputy National Statistician ONS, requested support from the Office for Statistics Regulation (OSR) for reclassification of MSQR to experimental statistics in advance of the publication of the August MSQR. Ed Humpherson, Director General for Regulation, wrote to Iain Bell to agree this reclassification. This request was due to limitations identified with the current estimates and the ongoing research programme ONS had in place to improve these statistics.

1.8 Following the pandemic, ONS announced in August 2020 that it would not be returning to producing official migration statistics using the IPS and instead, would be shifting to a new administrative data approach it terms admin-based migration estimates (ABMEs). This meant that the estimates would be based on actual patterns of migration that are evident from administrative data sources rather than on replies to survey questions about respondents’ intentions to remain in or out of the UK over the next 12 months.

Strategic and Tactical approaches

1.9 Currently ONS is applying two distinct methods in producing migration estimates. The first approach, which ONS is calling the tactical model-based approach, has been developed alongside experts at Southampton and Warwick Universities and can provide more timely, provisional estimates based on applying a statistical modelling technique known as a state space model to data sources, including the historical IPS data, Home Office border crossing data, visa data, and Advanced Passenger Information (API) data.

1.10 The second approach, which ONS is calling the strategic approach, uses DWP Registration and Population Interaction Database (RAPID) and Home Office visas and border administrative data sources. ONS looks at whether a migrant appears in the UK through one of these data sources and whether they have been here for 12 months (as the UN definition of a migrant is an individual ‘who has resided in a foreign country for more than one year’). This approach is not as timely as the tactical model-based approach, but it is hoped will be more accurate as it is based on observed behaviour.

1.11 ONS currently uses the tactical model-based estimates for its headline estimates of international migration, which are published as experimental statistics, and provides updates on progress with developing the strategic approach. In future ONS aims to combine the two approaches to produce timely provisional statistics which are then followed by more accurate final statistics. The statistical model itself is still evolving though and may change. For example, one area ONS is exploring is the feasibility of using Advanced Passenger Information (API) data to better support the provision of monthly migration statistics. The future development of the dynamic population model (discussed in more detail in 2.12) will also be informed by and help to inform the development of migration statistics.

Wider context

1.12 The migration transformation programme is part of a wider population and migration transformation programme which is seeking to understand the potential of administrative data sources and modelling to produce more regular and timely population and migration statistics at both national and local levels. Census 2021 results in England, Wales and Northern Ireland will also feed into the validation of the programme’s outputs.

1.13 In 2023, the National Statistician will report to the government on ONS’s recommendation on the future of the Census and population statistics in 2023. This will include an update on progress with its population and migration transformation programme, and will set out what is needed in the future to continue to achieve these ambitions.

How this part of the review was carried out

1.14 We have undertaken desk research looking at migration estimates in the UK and methods used to produce migration estimates, both here and abroad. We have held meetings with producers and key stakeholders with an interest in migration to capture their views about ONS’s current migration publication and its plans. We have spoken to academics, some of whom are involved in working with ONS, think tanks, devolved administrations, media commentators and other government departments.