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Volume 702: debated on Monday 16 June 2008

asked Her Majesty's Government:

What is their assessment of the reliability of their estimate of the population of the United Kingdom. [HL3904]

The information requested falls within the responsibility of the UK Statistics Authority. I have asked the authority to reply.

Letter from Colin Mowl, Director of Macroeconomics and National Accounts, to Lord Tebbit, dated June 2008.

The National Statistician has been asked to reply to your Question on our assessment of the reliability of our estimate of the population of the United Kingdom. I am replying in her absence. (HL3904)

The accuracy of the United Kingdom (UK) mid-year population estimate is dependent on the quality of data available to measure components of population change (births, deaths and migration). International migration is the hardest component to measure.

Of the data sources used to estimate population estimates:

the Census provides a reliable base; estimates of the reliability of the latest Census have been published and are available here: www.statistics.;

birth and death registrations are considered to accurately reflect numbers of events occurring in this country; and

international migration is difficult to estimate, though good use is made of available sources, but estimates are subject to a margin of confidence (as is discussed below).

The principal source of international migration data is the International Passenger Survey (IPS). As with all surveys, the IPS is subject to sampling variability. Standard errors, a measure of how much a sample estimate differs from the true value because of random effects, can be calculated from IPS estimates.

All sources also have non-sampling errors, such as non-response and errors in the answers given by respondents to surveys. In common with all national statistics institutes that estimate population change from a census, the ONS does not currently provide a single error measure to summarise the sources of uncertainty in estimation.

Each time data become available from a new census, the ONS is able to review the accuracy of population estimates made since the previous census by using the newly available information. The comparison that followed the census in 2001 highlighted the difficulty of estimating migration accurately.

It is considered that errors accumulated particularly in the latter half of the decade as the volume of migration flows increased in the late 1990s.

The need to improve migration and population estimates and provide more information on their accuracy has been recognised by the ONS for some time. In particular, the National Statistician commissioned an interdepartmental task force to look at how international migration estimates could be improved. It made recommendations in five broad areas to improve the compilation and presentation of international migration statistics, as follows:

improving the data available on numbers entering and leaving the United Kingdom;

making effective use of new and existing administrative and survey data sources;

improving local population estimates and projections used in allocating resources and developing services;

improving the public reporting of population and migration statistics; and

establishing a wider range of timely indicators and analysis to inform the evidence base on migration and its impacts on policy and public services.

On 4 February 2008, the Minister for Local Government announced to the House that a cross-government programme would be put in place to improve population and migration statistics, driven by senior officials from central government and the Local Government Association, and led by the National Statistician. It will take forward the recommendations of the 2006 interdepartmental task force on migration statistics.

This is high priority work for the ONS and other government departments and is being progressed with due pace. Future progress will be reported through the improving migration and population statistics page on the website:

Some changes have already been put in place. For example, the sample size of the International Passenger Survey was increased to collect more information on the flows of emigrants and further changes have been made to the IPS to enhance the coverage of migrants as they enter or leave the UK. Improvements were also implemented in 2007 to the methodology used for distributing migrants to the local authority level in the mid-year population estimates.