Census population estimates were published in September 2002. Since then, the Office for National Statistics (ONS) has been carrying out a number of studies into the reason for the difference between those estimates and the mid-year population estimates (MYEs) rolled forward from earlier censuses.
Initial research revealed two main causes of the difference. The mid-1991 estimates because ONS had made too big an adjustment for under-coverage in the 1991 Census. Errors in the estimation of net migration to and from the UK in the 1990s which caused ONS to over-estimate population change.
These factors accounted for about 60 per cent of the difference. ONS then undertook further investigations into the causes of the remaining unexplained difference. The results are outlined in the Downloads section.