Dear Mr Wragg,

While giving evidence to your Committee on 18 May, Alex Chisholm, Permanent Secretary at the Cabinet Office, asked that I write on his behalf regarding how we will ensure a coherent and consistent UK wide picture despite Scotland conducting their census in March 2022. I am very happy to provide an update.

Before providing that information, I would like to personally highlight to the Committee that the response to Census 2021 has exceeded all expectations, with returns from 97 per cent of addresses across England and Wales now having been received, surpassing our pre-census target of 94 per cent. The response has been similarly impressive in Northern Ireland with a current return rate of 97 per cent as well. Our next steps include the Census Coverage Survey, which will improve the quality of our final estimates, and our non-compliance operation. We set out in the Census White Paper ‘Help Shape our Future’ that we would lay the initial census results before Parliament within a year of Census Day, and we remain on track to do so.

Moving onto the specific questions around comparability of population data across the UK, there are two aspects to note.

Firstly, as you will no doubt be aware, we produce annual UK population estimates, which are rebased every 10 years based on new information from the latest census. As I previously reported to the Committee in September 2020:

“Certainly when the first indications were made that Scotland was considering the
decision that it has now taken, we brought to our UK census committee a paper that
looked at how we would overcome the problem of re-basing our population estimates
one year apart. I am confident that the strategies that are being proposed to do that
are robust and will not unduly impact on the accuracy of population data over the
next decade.”

The different census dates for the UK will mean that the rebasing process will differ from previous approaches, while ensuring the figures remain comparable. This time, the rebasing process for each country will be staged according to the year in which the census takes place. For England and Wales, the production of the mid-2021 population estimates will be based on 2021 Census data, while for Scotland, they will be rolled forward from mid2020 population estimates. These will feed into the UK population estimate for mid-2021.

We then expect to produce a rebased series of UK population estimates in autumn 2022, covering the period between Census 2011 and Census 2021, which will comprise of census based population estimates for all UK countries bar Scotland, which will again be rolled forward population estimates. In this way, population estimates for the UK for 2021 and 2022 will be based on the latest most accurate information available for each of the constituent countries, whether Census based or rolled forward at the particular time of production, and each using the same definition of usually resident population.

Secondly, there are considerations in respect to the census outputs produced, including UK combined outputs. The census offices continue to work closely together in particular to ensure harmonisation of concepts and definitions wherever possible to support comparability across the UK. These principles are set out in the public agreement between myself and the Registrars General for Scotland and Northern Ireland.

The move to a March 2022 census date in Scotland will mean some changes in the outputs possible compared to those produced following the 2011 Census. We have established a UK Census Data User Working Group to actively engage with UK census users to design the required outputs and explore the possible statistical techniques to provide combined outputs. The work acknowledges the possible rapid change in some characteristics between 2021 and 2022 given the pandemic and other factors. Alongside all outputs we will provide supporting information and guidance to assist users in interpreting combined UK data outputs.

More generally, we will shortly be carrying out a formal consultation on the full range of 2021 Census outputs we plan to produce to ensure that the products made available will best meet the needs of users, especially those users of UK census outputs.

Yours sincerely,
Professor Sir Ian Diamond