Enumeration
The overall response rate for the census, that is the proportion of the population who were included on a census form, was estimated to be 94% in England and Wales, 96.1% in Scotland and 95.2% in Northern Ireland. This was due to a number of factors: households with no response, households excluding residents from their returns, and addresses not included in the enumeration. In Manchester for example 25,000 people from 14,000 addresses were not enumerated because the address database was two years out of date. The Local Authority with the lowest response was Kensington and Chelsea with 64%. Hackney had the next lowest response at 72%. Out of all local authorities, the ten lowest response rates were all in London. The results still represent 100 per cent of the population, however, because some individuals not completing their forms were instead identified by census enumerators, and through the use of cross-matching with a follow-up survey.
Read more about this topic: United Kingdom Census 2001