This section provides comprehensive explanation of statistical methods and quality, and of the confidence which can be placed in the 2001 Census results both in general terms and more specifically for topics and local areas.
The One Number Census - an estimate of the whole population
2001 Census results are the first to represent the entire population, not merely those enumerated as in past Censuses. This was achieved through a new strategy known as the 'One Number Census'. A key elements was an independent follow-up survey - the Census Coverage Survey (CCS) which involved face to face interviews with 320,000 sample households spread across every local authority in the UK. By combining the results of the Census and the CCS, it was possible to estimate the total resident population in 2001 - the 'one number' - to a high level of precision.
National and local response rates
While the 2001 Census results represent 100 per cent of the population, the total overall response rate was 98 per cent. This includes some 4 per cent of the population estimated to have been resident in households which did not return forms but were identified by enumerators. The proportion of people returning a form in England and Wales was 94 per cent.
The accuracy of the 2001 Census results
The results of the 2001 Census are the most accurate ever. By combining the results of the Census itself and those from the Census Coverage Survey, and by carrying out extensive quality checks, the total population of England and Wales is accurate to within +/- 0.2 per cent.
One Number Census imputation rates by key variables
Under-enumeration in the 2001 Census did not occur uniformly across all areas. Response rates were lowest for inner city areas where characteristics known to be related to non-response are most prevalent.
This section provides an imputation rate for each local authority based on the numbers of people and households in each local authority that were imputed into the final counts, analysed by the key questions asked on the Census form.
Quality indicators for local areas
Quality indicators for all local authorities, wards, Output Areas, Parishes and postcode sectors were derived from the proportion of people imputed in the area expressed as a four point scale. The indicators have also been produced for each Strategic Health Authority, Primary Care Trust and Health Board (Wales).
Variability in tabular counts
A large scale data collection exercise like the Census is never 100 per cent accurate, and some variability in the final results would be expected. This means that there may be small differences between the notional 'true' counts in a population and the estimated counts that are released. The 2001 Census was no exception, and there were many reasons for variability to occur.
A note provides information on the sources of variability, and, where possible, a guide to the level of variability that may occur within tables at different geographical levels.
There is additional special guidance on variability in tables of flows between residence and workplace and of migrants - the origin/destination matrices.
Local authority information pages
This section brings together the measures which indicate the quality of the 2001 Census for each local authority in England and Wales.
Census Quality Survey
This report of the Census Quality Survey provides information about some aspects of the quality of the 2001 Census. The Census 2001 Quality report for England and Wales, published on Nov 29th 2005, provides a broader range of information about Census quality, and includes summary information about the Census Quality Survey. The Survey was carried out in 1999 to provide information about the accuracy of responses to the Rehearsal version (1.73MB pdf file) of the 2001 Census form. A sample of households were asked to complete the form and interviewers later called on respondents to check the accuracy of their responses to the form and record comments on how the questions were answered. This report provides a basis for understanding the reasons for incorrect answers and highlights the impact which the CQS had on reorganising the questions on Qualifications and Carers for the main 2001 Census.
The Census exchange - an international perspective
A reference site for researchers in Census methodology was setup during the 2001 Census, with papers and articles providing an international perspective on census taking and has been retained for reference.
Census News
