The Census response
In 1991, the estimated coverage and total overall response of the census in England and Wales was 98 per cent. This was the proportion of the population accounted for in the census results. It included some 2 per cent estimated by enumerators to be resident in identified households but from whom no completed census form was collected. Census response in 1991, defined as the proportion of the population counted on returned census forms, was 96 per cent in England and Wales.
Census response in 2001 for England and Wales was estimated to be 94 per cent, 2 per cent lower than in 1991. This decline in response rates is in line with changes that have been observed for large-scale Government Surveys and censuses conducted in other countries in recent years.
Under enumeration in the 2001 census did not occur uniformly across all areas or age-sex groups. The patterns of census response were as expected, that is response rates were lowest for persons in their twenties, particularly men, and for inner city areas where characteristics known to be related to census non-response are most prevalent - multi-occupancy, language difficulty, etc. 2001 census response by age-sex group for England and Wales as a whole varied from 98 per cent for females aged 75-79 to 87 per cent for males aged 20-24.
Adjusting for overcount
Although the ONC methodologies are principally concerned with identifying and adjusting for census under-enumeration, part of the CCS interview was also aimed at identifying any potential overcount in the 2001 census, that is persons incorrectly enumerated as resident at more than one address. A number of factors built into the process helped to reduce the chance of over-enumeration. These include simplification of definitions and clarification of who should be included and where. The expectation is that over-enumeration will only occur accidentally. Examples of such possibilities include second homes and children from broken homes living a proportion of time with each parent. Analysis of responses to the CCS indicated that the level of overcount in the 2001 census was negligible - less than 0.1 per cent of the population were estimated to have been counted twice. This is well within the estimated precision of the ONC estimates (plus or minus 0.2 per cent for England and Wales) and has therefore been taken as not significant in calculating the ONC estimates.
Precision of the One Number Census results
The quality assurance process means that the census figures are the best estimates we can make of the population. However, they are estimates and therefore subject to margins of error. Standard statistical techniques have been used to calculate these error levels, and therefore produce confidence intervals for the ONC results. A 95 per cent confidence interval is a range within which the true population would fall for 95 per cent of the times the sample survey was repeated. For England and Wales as a whole, the confidence interval on the population estimate is +/- 0.2 per cent. This means that the total census figure has a margin of error of plus or minus 104,000. For local authority areas the percentage margins of error are larger, ranging from 6.1 per cent in Luton to 0.6 per cent in Dudley, East Dorset and Redcar and Cleveland. This is the first time it has been possible to estimate the level of precision for a census with any confidence. It should be noted that as with all statistical analysis these standardised calculations do not capture all sources of variation and there will also be, for example, response, capture and coding errors - see the 2001 Census Quality Report. However, our assessment is that, having made an adjustment for dependency, the ONC results remain the best central estimates possible of the population as at census day 2001.