The One Number Census - an estimate of the whole population
Census 2001 was the most accurate count of the UK population ever achieved
Census 2001 results are the first to represent the entire population. This was achieved through a new strategy known as the 'One Number Census'(ONC). One of the key elements of the ONC 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 per cent. 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.
Following evaluation of the 1991 census a number of new initiatives were introduced to maximise coverage. These included encouraging people to post back their census forms to enable census staff to focus on those most likely to have difficulty filling in their forms, smaller workloads for staff in the more difficult areas, redesigned and carefully tested forms and questions, a community liaison programme including translation of census material into 26 languages, and a focused programme of awareness raising and publicity.
We knew, however, that we would not get 100 per cent response and indeed expected that undertaking the census would be more difficult in 2001 than it had been in 1991. The pattern of increasing difficulty of obtaining response to a census was also evident in many other countries. In addition we were aware that in 1991 the survey we conducted to estimate for those that were missed was not of a large enough scale to fully identify the extent and distribution of the under-enumeration. As a result, it was necessary to base the national population estimates for 1991 on demographic estimates. This in turn led to low level census counts which did not add to national and local authority level totals.
Users were clear that for 2001 they wanted us to produce a fully adjusted set of counts covering 100 per cent of the population. The One Number Census project (ONC) was set up to meet this need. By conducting a redesigned and much larger post-enumeration survey (the Census Coverage Survey, CCS) and combining the results of both the census and CCS in what is known as a dual system approach, the aim of the project was to estimate, and adjust the census database for, under-enumeration so that all statistics add up to 'One Number' and also to ensure that robust results could be obtained for each local authority area.
The ONC methodologies were researched and developed over a number of years by a joint team of statisticians from the ONS and the University of Southampton. The work was overseen by a Steering Committee that included experts from central government, statistical agencies overseas, other academic institutions and local government. Research papers were produced for each meeting of the group. These papers contained the detailed research undertaken throughout the project and the details of the final methodology.
The remaining Steering Committee papers can be found below, grouped by year. Please note that there may be more than one paper on a particular subject, written at different stages of the development of the methodology under discussion.
In addition, census users were consulted at several stages in the methodological development process through census user group meetings and special workshops.
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