Dear Mary,

I am writing to provide updates on two recent pieces of regulatory work that we have undertaken in relation to population statistics produced by the Office for National Statistics (ONS).

Confirmation of accreditation of the 2021 Census in England and Wales outputs (excluding gender identity statistics)

We have reviewed the actions that ONS has taken to address the requirement outlined in our third and final assessment report on the 2021 Census in England and Wales. On behalf of the Board of the UK Statistics Authority, I am pleased to confirm that these statistics comply with the standards of Trustworthiness, Quality and Value in the Code of Practice for Statistics (the Code) and should continue to be labelled as accredited official statistics. The detail supporting our judgement is set out in the annex to this letter. I would like to thank the ONS teams involved in Census 2021 for how they have responded to the requirement and our other recommendations.

ONS’s report ‘Impact of the coronavirus (COVID-19) pandemic on England and Wales Census 2021 outputs’, published in November 2025, addresses our requirement by setting out which Census 2021 estimates may have been affected by the pandemic, why and how users should interpret them, alongside providing appropriate signposting to alternative sources.

However, we found that ONS provides only very limited justification for its confidence that Census 2021-based mid-year population estimates in England and Wales – which we have also reviewed (see below) – are free from systemic pandemic-related effects. Further assurance and clearer explanation of remaining uncertainties are therefore essential. We will monitor ONS’s progress in improving this essential assurance through the follow up of our mid-year population estimates compliance review.

In response to the recommendations in our assessment report ONS has taken several positive steps to strengthen transparency, accessibility and user understanding across its census outputs. This has included providing clearer and more consistent communication of revisions, corrections and quality information. ONS is also making progress in helping users more easily find and appropriately interpret statistics through its wider website transformation programme, with improved navigation, clearer signposting to quality and methods content, and enhanced explanatory features within articles. We also note ONS’s continued commitment to effective user engagement and collaboration.

Compliance review of ONS’s mid-2024 population estimates (MYEs) for England and Wales

Today we have published the findings of our compliance review of ONS’s mid-year populations estimates for England and Wales against the standards of the Code. The review considered whether these statistics continue to meet the standards of Trustworthiness, Quality and Value and whether they should retain their accredited official statistics status.

These statistics provide the official annual measure of the population in England and Wales, offering essential insights into population size and distribution that underpin a range of economic statistics and inform key national and local operational, financial and policy decisions. Our review concludes that MYEs continue to provide an important and valued source of population insight.

However, our review sets out our significant concerns about the quality of subnational estimates, particularly in local authority areas that are subject to high population churn.

Our review identifies four requirements which must be met for the MYEs to retain their status as accredited official statistics. These requirements aim to strengthen user confidence as ONS focuses on improving the traditional cohort component-based MYEs and confidence in the Census 2021 baseline that underpins them, improve users’ understanding of the strengths and limitations of the estimates, and enhance the guidance that supports their appropriate use. ONS must update us on how it has met these requirements before or at the same time as its next planned publication of the MYEs in summer 2026.

Our review also identifies several further recommendations to enhance the Trustworthiness, Quality and Value of these statistics. The recommendations focus on strengthening user confidence in ONS’s improvement plans and estimates, enhancing quality assurance and addressing user concerns related to revisions and publication delays. We expect ONS to report publicly to us on its progress in relation to these recommendations by the end of 2026. Given ONS’s focus on the continuous improvement of the MYEs, our review findings should also play a central role in shaping planned work.

I am grateful for the constructive engagement from your teams throughout these reviews.

I am copying this letter to Stephanie Howarth, Chief Statistician Welsh Government, and the Statistics Head of Profession Office at ONS.

Yours sincerely,

Ed Humpherson
Director General for OSR

 

Annex A – Review of actions taken in response to the Assessment of compliance with the Code of Practice for Statistics: 2021 Census in England and Wales

Requirement 1: To support user confidence and the appropriate and ongoing use of census data affected by the pandemic (including where census data are used as a data source in other statistics, such as the annual mid-year estimates), ONS should:

• provide more detail in its guidance to help users understand what weight can be placed on affected statistics in decision making.
• signpost to other relevant topic statistics available for 2021, beyond those based on census, that could also provide users with a more holistic view of a particular topic.
• explain, by census topic and geographic breakdowns, which estimates may be affected and why. For local authorities in England and Wales, the information provided should include an analysis that identifies factors or characteristics of an area that may lead to increased uncertainty for that area.

Progress made by ONS: On 6 November 2025, ONS published a report titled ‘Impact of the coronavirus (COVID-19) pandemic on England and Wales Census 2021 outputs’. The report sets out how the pandemic affected key Census 2021 topics and highlights implications for use of these statistics. The report also explains that census data form the basis for mid-year population estimates (MYEs) and provide a benchmark against which the quality of its admin-based population estimates (ABPEs) can be assessed.

Taking each part of the requirement in turn:

1) ‘Provide more detail in its guidance to help users understand what weight can be placed on affected statistics in decision making’

In its November 2025 report, ONS provides information about implications for use. This information uses language like “take care when using” and “factor in” to help users understand how to use the data. The impact report is also now signposted within relevant quality reports, such as the MYE quality and methodology information (QMI) and the Census QMI, to help users find it.

2) Signpost to other relevant topic statistics available for 2021, beyond those based on census, that could also provide users with a more holistic view of a particular topic

For each topic ONS sets out possible impacts, other data sources and implications for use, for example, Greater London Authority’s report, Population change in London during the pandemic, and Higher Education Statistics Agency’s (HESA’s) report, The impact of the COVID-19 pandemic on 2020/2021 Student data.

3) Explain, by census topic and geographic breakdowns, which estimates may be affected and why. For local authorities in England and Wales, the information provided should include an analysis that identifies factors or characteristics of an area that may lead to increased uncertainty for that area.

ONS’s November 2025 report sets out, by census topic, the extent to which estimates may have been affected by the pandemic. It explains that some topics were more affected than others and highlights those urban areas and local authorities with higher population churn, such as areas with large student populations, are more likely to experience increased uncertainty.

ONS did not identify specific local authorities that are most affected, noting that in principle all areas were affected by the pandemic. Instead, it outlined the characteristics associated with higher levels of uncertainty.

ONS told us it is continuing to do more research to better understand movement to ensure the estimates are fit for the future. ONS is researching how supplementing its internal migration methods with tax and benefit data may improve the lag and accuracy issues of migration, particularly that of students.

OSR view: In summary, ONS’s report ‘Impact of the coronavirus (COVID-19) pandemic on England and Wales Census 2021 outputs’ addresses this requirement. The report explains that Census 2021 provides a unique snapshot of the population during the pandemic, which may differ from non-pandemic conditions for some topics.

ONS’s research also shows that ABPEs, even when not coverage-adjusted by census, showed good comparability to 2021 MYEs (2021 census-based). A total of 307/331 local authorities were within 3.8% of the MYE population, a target quality threshold ONS held itself to. There was some drift, but ONS says it rigorously describes the shortfalls of the admin-based system, such as changes in patient register completion during the pandemic, and delays to birth registrations. ONS sees this as an independent measure of population that corroborates with census estimates, implying the measures, even at local levels, are accurate, despite Census 2021 being undertaken during a pandemic.

However, separately ONS needs to do further work to provide assurance in its confidence that MYEs do not have systemic pandemic effects in the years since 2021. While ONS states that census-based MYEs are robust and free from systemic pandemic effects, it provides limited justification for this confidence. As this issue relates to onward use rather than concerns about the quality of the census statistics themselves, we will close this requirement and take this issue forward through OSR’s compliance review of the mid-year population estimates.