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Introduction to population estimate revisions

Revised mid-2001 population estimates were published on 26 September 2003. About 190,000 people have been added to the mid-2001 estimate of the population of England and Wales, revising it to 52.3 million. Most of the extra people are young adult men aged between 25 and 34. The mid-2002 population estimates which are also published today incorporate the revision to the mid-2001 estimates.

See Related links for the latest population estimates.

Since the Census population estimates were published in September 2002, the Office for National Statistics (ONS) has been carrying out a number of studies into the reasons for the difference between the 2001 Census based estimates and the mid-year population estimates (MYEs) rolled forward from earlier censuses. At the time the mid-2001 estimates were published, the 1.1 million overestimate for England and Wales was attributed to two factors:

  • the mid-1991 population estimates were too high because they included too big an adjustment for under-coverage in the 1991 Census. 1991 MYEs for England and Wales were published on 13 February 2003 revised down by 351,000

  • errors in the estimation of net migration to and from the UK during the 1990s caused estimation of population change to be too high. Migration is generally recognised as being difficult to estimate accurately due to the rarity of high quality data sources, particularly emigration

A National Statistics Quality Review of International Migration had already been initiated because of weaknesses in the statistics on migration. Revised migration estimates for 1992-2001 were published in June 2003. These showed a further 305,000 overestimate in the rolled-forward population estimates. Therefore, the two main factors that caused the rolled forward estimates to be too high accounted for over 650,000 (about 60 per cent) of the 1.1 million difference.

Further research included demographic analysis of sex ratios, fertility, mortality and migration, analysis of the Longitudinal Study, comparisons with a range of administrative sources, investigation of Census data and processes, and matching studies of address lists held by Local Authorities and those collected by the Census. The key findings from these studies, not all of which are completed, are described in separate documents found in Evidence and analysis.

The evidence from the various studies indicates a possible underestimate of men, particularly young adult men aged between 25 and 34, in the 2001 population estimates and to a lesser extent men aged 35 to 44. Given the weight of this evidence it was decided to revise the mid-2001 population estimates to add around 190,000 to the population of England and Wales, most of the added people are young adult men aged between 25 and 34. A small adjustment was also made to the 35 to 49-year-old age group partly to reflect the evidence and partly to help create smooth demographic rates.

This also reflects some residual effect, predominantly from men aged from 25 to 34, in earlier years. The revision to the mid-2001 population estimates explains a further 190,000 of the difference. The explanation of the difference between the 2001 Census based mid-2001 population estimates and the rolled-forward population estimates is summarised numerically in the table below:

Intercensal Discrepancy


Of which:

Change to the 1991 base

Revised Migration Estimates

Revisions to 2001





Leaving Unattributed Change



Estimates for Scotland and Northern Ireland are the responsibility of the General Register Office for Scotland (GROS) and NISRA respectively. GROS and NISRA have published their 2002 MYEs and are content that the MYEs in Scotland and Northern Ireland do not require revision either for 2002 or earlier years. If new information emerges which indicates a need for a change to the MYE, the information will be considered in the next round of MYEs for Scotland and Northern Ireland respectively.

What happens next

The population estimates published on 26 September taking account of all the available evidence from existing statistical sources. These estimates are suitable for the full range of uses, including resource allocation.

Some studies, notably the matching exercises, have not yet been concluded. Results of the Manchester exercise are expected to be available by the end of October 2003. Results of the Westminster matching exercise will follow. A third stage, in Spring 2004, will review the outcomes from both matching studies and whether any further revision may be required for any area. Also following publication of the review: 'A demographic statistics service for the 21st Century', ONS will outline an action plan that will lead to continuing improvements in the reliability of population statistics produced between Censuses.

Some of these actions may lead to the need to further revise population estimates, as improved methods and sources are put in place. ONS will be consulting widely on the programme of methodological research.
The implication of revisions to mid-2001 is that some revisions will also be required to the historic population series for the years 1992 to 2000. ONS will publish an interim revised series for England and Wales, by age and sex, in October 2003. It is planned that comprehensive revisions to the historic series will be published in Spring 2004.

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