Revisions to the Population Estimates

The 2001 Census provides the benchmark for a new decade of population estimates. More about the Census.

The mid-2001 population estimates were based on the Census. These estimates showed a 1.1 million difference in England and Wales compared to estimates rolled forward from previous censuses. Part of the difference has been explained by the 1991 base for the population estimates being too high and part due to errors in estimating migration accurately due to difficulties that are well known. These accounted for 60 per cent of the difference. ONS undertook an investigation into the remaining unexplained difference which included research to give better understanding of the census data.

It has been recognised that, at a time of increasing geographic mobility and diverse family structures and living arrangements, certain population groups may have been particularly difficult to measure. The Census was designed with this in mind. Analyses suggested that it sucessfully captured the vast majority. New investigations suggest a small group (0.4% of the population) for which a revision is needed to Population Estimates.

• What happened and how ONS has addressed the issue
• Detailed reports of ONS's analyses to explain the difference
• ONS's analyses included investigation of Census data and processes
• ONS has revised mid-2001 population estimates for men aged 25-49, particularly for those aged 25-34. Summary of the impact and implication of these revisions.
• How the revisions were made
• What happens next

This page last revised: Friday 26 September 2003