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.
The Office for National Statistics (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 the population estimates.
Overview of revisions to ONS's population estimates from 2003
Evidence and analysis for differences in 2001 Census mid-year population estimates with earlier estimates
Summary of the impact and implications of revisions to mid-year population estimates (MYEs) from the 2001 Census
Future plans for ONS mid-year population estimates, 2003