Council and democracy

How accurate is the 2001 census?

The information presented here is extracted from 2001 Census Definitions, Crown copyright 2004

Quality assurance

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 Census 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, for example, 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. The corresponding confidence interval for Scotland is 0.3 per cent (+/- 17,000), and that for Northern Ireland is 0.7 per cent (+/- 12,000).

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 - these will be outlined in full in the 2001 Census Quality Report.

Census response

In 1991, the response to the Census (that is, the proportion of the population counted on returned Census forms) was estimated to be 96 per cent in England and Wales; 97 per cent in Scotland; and 98 per cent in Northern Ireland. In England and Wales, and Scotland, information collected by enumerators for households where no completed Census form was collected was taken account of in the results, increasing the coverage of the Census (that is, the proportion of the population covered in Census results) to an estimated 98 per cent in both Scotland and England and Wales. This process was not applied in Northern Ireland.

In 2001, coverage of the published Census results is 100 per cent for the whole of the UK. The response, as measured by the proportion of people enumerated on Census forms, was 94 per cent in England, 96 per cent in Scotland and 95 per cent in Northern Ireland. This decline in response rates is in line with changes that have been observed for large-scale Government surveys and censuses. In order to get 100 per cent coverage a Census Coverage Survey (CCS) was designed and conducted independently of the Census during May/June 2001. Records from the CCS were matched to those from the 2001 Census. Populations of the sample areas were estimated from the results of the matching using dual system estimation techniques, which enabled an estimate of those persons missed by both the Census and the CCS to be made. Populations for each local authority (or NUTS III area in Northern Ireland) by age and sex were then estimated using a combination of standard regression and small area estimation techniques. Households and persons estimated to have been missed by the Census were then imputed to produce a fully adjusted Census database. The characteristics of these households and individuals were modelled using the returns from the Census and CCS; and finally all population estimates were carefully quality assured using demographic analysis and comparison with aggregate level administrative data.

The spreadsheet below details response rates from the census for particular areas: