Dear Mary,
Thank you for your letter in April providing an update on how the Office for National Statistics (ONS) has been developing its long-term international migration (LTIM) estimates. Your letter, alongside ONS’s latest update on its international migration research, demonstrates the considerable progress that ONS has made in improving admin-based migration estimates, while recognising that this work continues to develop.
In December 2023 we reported on ONS’s progress against the recommendations in our 2022 Review of Migration Statistics. At that time, there were four recommendations still to be addressed:
- developing plans for the ongoing review of data sources
- developing and publishing measures of uncertainty
- publishing long-term plans for international migration
- publishing a road map for population and migration transformation
We have reviewed the actions ONS has taken to address the remaining recommendations, and I am pleased to confirm that we judge them to be met and will therefore close this review. The detail supporting our judgement is set out in the annex to this letter.
ONS has made considerable progress improving its long-term international migration estimates, including moving from the International Passenger Survey to administrative data sources, strengthening methodological assurance and transparency, developing uncertainty measures, and regularly engaging with users. We expect ONS to build on this progress, by continuing to research and test methods to improve the estimates; address remaining gaps, such as estimates for non-UK-born populations; and maintain user confidence in the relevance and quality of migration statistics. Once ONS considers the LTIM estimates are ready, my team would be happy to discuss timings for an assessment with you, to determine whether accredited official statistics status can be granted.
We would like to thank the ONS migration team for its productive engagement throughout our review. I am copying this letter to the Statistics Head of Profession Office at ONS.
Yours sincerely,
Dr Anna Price
Assessment Programme Lead, OSR
Annex – Review of actions taken in response to the remaining four recommendations from OSR’s 2023 progress report on ONS’s migration estimates
Recommendation B: ONS should ensure data sources used in the estimates remain up to date. It should be clear with users how it will achieve this, for example by identifying points when it will review the data sources, such as use of historical IPS data and Home Office visa data.
Action remaining from December 2023: ONS should publish information on how it will ensure that its estimates remain up to date and how it will review data sources going forward.
Actions taken by ONS:
ONS continues to publish regular methods updates and quality information for each of the data sources used to compile the estimates as part of the release.
The Advisory Panel on Migration Statistics was established in mid-2025 to provide independent advice to the National Statistician to help prioritise developments of migration statistics based on user needs and available resources.
ONS has developed a research maturity framework to provide structure to and quality-assure any research, including the exploration of new data sources, through distinct phases with clear oversight and review. As part of this framework, research is reviewed by internal (Methods and Research Assurance Group (MaRAG)) and external assurance groups (Methodological Assurance Review Panel Migration Statistics sub-group (MARP)), with research papers published. ONS publishes research updates to inform users of progress, including the investigation of new sources and methods.
OSR evaluation of evidence:
ONS has put formal structures in place to ensure that new methods and data sources are suitably researched and tested, and that users are well informed of any upcoming changes. The use of defined development phases, supported by internal and external reviews, helps ensure that methodological changes are rigorously assessed before implementation and are informed by a range of expert inputs.
ONS has also improved the transparency of its development process through regular publications and research updates. These outputs provide users with clear explanations of methodological changes, including the transition away from IPS data towards administrative sources. By setting out both the rationale for these changes and planned future developments, ONS is helping users to understand how data sources are evolving and why.
We look forward to ONS continuing to build on this progress, ensuring users are consistently informed about the quality and relevance of migration estimates.
Judgement: We consider this recommendation to be met.
Recommendation E: Users need a clear understanding of uncertainty associated with the estimates and how the estimates can be used appropriately. ONS should share detailed plans for how uncertainty will be measured and communicated.
Action remaining from December 2023: ONS should publish further work on uncertainty to enable appropriate use and avoid misuse of the LTIM estimates.
Actions taken by ONS:
ONS has developed new uncertainty estimates for administrative data sources. Several versions of these estimates were created and tested against each release and iteratively improved via expert feedback from the MaRAG and MARP. In the published estimates (May 2025), ONS provided, for the first time, upper and lower bounds for immigration, emigration and net migration.
ONS has documented its progress through regular methods updates and papers on measuring and quantifying uncertainty in international migration estimates. ONS has also published work on modelling revisions to understand estimate stability and confirmed plans to use a simulation-based approach for uncertainty intervals. Recent releases have strengthened user guidance, including a new section on how the estimates should and should not be used. The May 2026 release includes a new chart showing simulation-based uncertainty estimates, explaining clearly what components of uncertainty the estimates do and do not include. ONS is also developing a new approach to understanding uncertainty based on a precocity framework, with further methodological details published on estimate stability.
OSR evaluation of evidence:
ONS has made clear and sustained progress in developing and communicating measures of uncertainty in its long-term international migration estimates. Since our previous update, ONS has moved beyond initial exploratory work to implementing and testing multiple approaches to quantifying uncertainty.
The addition of simulation-based uncertainty estimates in the May 2026 release represents further meaningful progress, particularly as these are accompanied by clear explanations of what sources of uncertainty are and are not included, which helps support appropriate interpretation and use.
ONS has also strengthened its supporting materials. For example, it has documented methodological developments, including work on quantifying uncertainty and modelling revisions to understand estimate stability, through a series of updates and papers. The introduction of a dedicated section, “How to use these international migration estimates”, in the November 2025 release is a particularly positive step in helping users understand appropriate use, limitations and potential risks of misuse.
ONS has demonstrated good practice in engaging users throughout this process, including seeking feedback on uncertainty estimates and signposting to its wider research. The inclusion of clear visualisations, such as the new uncertainty chart, improves accessibility and aids user understanding.
Judgement: We consider this recommendation to be met.
Recommendation F: ONS should provide clearer information about its longer-term plan for measuring migration estimates. For example, what the longer-term model will look like and how the state space model method (tactical approach) and the admin-based method (strategic approach) will be integrated with this longer-term model and how this fits with ONS plans for a Dynamic Population Model.
Action remaining from December 2023: ONS should publish its longer-term plan for migration statistics and keep users informed of its progress.
Actions taken by ONS: ONS has published its longer-term plans through quarterly updates on population and migration statistics, such as the most recent May 2026 update. These quarterly updates are supported by methods updates published twice a year, setting out ongoing research and evolving methods. ONS keeps users informed through its regular newsletters, pre- and post-publication webinars and engagement with the National Statistician’s Advisory Panel and the Migration Statistics User Group, where long-term plans are discussed and recorded.
OSR evaluation of evidence:
ONS has made positive progress in improving how it communicates its plans for population and migration statistics through introducing its quarterly updates as well as regular methods publications. ONS maintains ongoing engagement with users via webinars and user forums such as the 2025 Migration Statistics User Forum, which took place on 11 December 2025, with the next user forum scheduled for December 2026. We welcome that these actions support transparency, enabling users to plan ahead, and provide clear routes for users to comment on ONS’s emerging priorities and methods.
In addition, ONS has strengthened its engagement with expert and user communities through the National Statistician’s Advisory Panel and the Migration Statistics User Group.
Judgement: We consider this recommendation to be met.
Recommendation K: ONS should publish a high-level roadmap for the population, migration, and labour market transformation programme, including how migration fits with the rest of the transformation plans, when migration outputs are due to be published and how users can get involved.
Action remaining from December 2023: ONS should publish a transformation roadmap which is regularly updated to reflect new information, including from the Future of Population and Migration Statistics (FPMS) consultation.
Actions taken by ONS:
ONS set out its prioritisation of economic and population statistics in its work plan for economic statistics, and provides OSR with regular updates on progress against the plan.
The transformation of migration statistics (as part of broader work on population statistics in ONS) was overseen by the Future of Population and Migration Statistics (FPMS) programme. The UK Statistics Authority published its recommendation on the future of population and migration statistics in England and Wales in June 2025, and the FPMS programme was closed. Work to plan and operationalise a 2031 census is now underway in England and Wales, as set out in ONS’s Census 2031 Strategy for England and Wales.
ONS publishes a 12-month look-ahead in its quarterly updates on population and migration statistics, and plans to publish a strategy for migration statistics in autumn 2026.
OSR evaluation of evidence:
The broader population statistics landscape has changed since our 2022 review. The confirmation of a 2031 census in England and Wales and ONS’s published strategy provide clarity and strategic direction on the future of population statistics. ONS has signalled its broader priorities for its statistics through related programmes, such as its economic statistics recovery plan, which includes its priorities for population statistics. These multiple strands demonstrate that ONS is progressing work across population, migration and labour market statistics and is taking steps to align these areas strategically.
ONS has improved the visibility of its transformation activity through its quarterly updates on population and migration statistics. Together, these outputs contribute useful context about the direction of travel and the increasing role of administrative data within the system.
Judgement: We consider this recommendation to be met.
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