In all previous censuses, the total population given by the census was the raw count, reflecting a response rate always short of 100% (98% in 1991 even after adding in the imputed absent households). The 2001 Census is the first to produce a 'count' (really an estimate) that represents the entire population. This was achieved through a new strategy known as the 'One Number Census' (ONC).
As in the past, a key element of the ONC was an independent post-enumeration survey, the Census Coverage Survey (CCS). This was a far bigger survey than the 1991 Census Validation Survey and involved face to face interviews with a sample of 320,000 households from every local authority in the UK. By combining the results of the census and the CCS, it was possible in 2001 to estimate the total resident population - the 'one number' - to a high level of precision, plus or minus 0.2 per cent. Thus the 2001 Census has generated a single national count of the population on census day, 58,789,194 to which all other census output will now sum.
For a summary of the One Number Census see http://www.statistics.gov.uk/census2001/onc.asp [Note: This link opens in a new window]
A detailed guide to the ONC is available at: http://www.statistics.gov.uk/census2001/pdfs/oncguide.pdf [Note: This link opens in a new window]