Census 2001 results are the first to represent the entire population. This was achieved through a new strategy known as the 'One Number Census'. One of the key elements was an independent follow-up survey. The Census Coverage Survey (CCS), as it is known, involved face to face interviews with a sample of 320,000 households from every local authority in the UK. In the past, the total population given by the census was the raw count, reflecting a response rate of 98%. But 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.
In 2001, the Census results represent 100 per cent of the population. Total overall response was 98 per cent. This includes some 4 per cent of the population estimated to be resident in households identified by enumerators. Census response, the proportion of people returning a form in England and Wales, was 94 per cent.
The results of the 2001 Census are the most accurate ever. By combining the results of the Census itself and those from a follow-up survey of 320,000 households and carrying out extensive quality checks, the total population of England and Wales is accurate to within +/- 0.2 per cent.
The UK has 800,000 fewer young men than previously thought. This was originally identified in the 1991 census, but given a lack of confidence in the follow up survey for that census, the numbers were revised to restore the predicted pattern. In 2001, the methods were so entirely robust that similarly surprising results must be accepted, and population estimates rebased back to 1982. The critical factor appears to be emigration. The International Passenger Survey works well, but it captures travellers' intentions at the time of departure. These may be prone to change once people are abroad, particularly among young men with few ties at home.
Quality indicators for all Local Authorities, Wards, Output Areas, Parishes and Postcode Sectors have been derived from the One Number Census person imputation rates. Quality indicators have also been produced for each Strategic Health Authority, Primary Care Trust and Health Board. The quality indicator consists of a four-point scale, relating to the proportion of imputed people in an area. The scale used is shown here:
No large scale data collection exercise will ever be 100 per cent accurate and we would expect some variability in the final published results. This means that there might be small differences between the 'true' counts in a population and the estimated counts that are published. The 2001 Census is no exception and there are many sources for variability to occur for person, household and communal establishment tables.
This paper sets out to describe some of these sources and where possible, presents a guide to the level of variability that may occur within published tables at different geographical levels.
Under-enumeration in the 2001 census did not occur uniformly across all areas. The patterns of census response were as expected, that is response rates were lowest for inner city areas where characteristics known to be related to census non-response are most prevalent such as multi-occupancy and higher proportions of non-English speaking population.
The numbers of people and households in each local authority that were imputed into the final counts were used to produce an imputation rate, which was split by the key questions asked on the Census form. [more details]
A number of measures have been produced to help users understand the quality of the 2001 Census for each local authority. These have been presented on the National Statistics website as they have become available.
This information has now been brought together for each local authority in England and Wales [more details]
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 (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.
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