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 complete coverage 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 most census taking countries it is standard practice to measure the level of census under-enumeration by either a post enumeration survey and/or by comparison of aggregate census counts with aggregate data from other sources such as health registers or benefit records. It is usual practice to produce a report assessing the level of census response for the purpose of re-basing a population estimate series, but the results of the census itself have never been adjusted for under-enumeration.
For the first time, however, all the 2001 census results in the UK have been adjusted for estimated under-enumeration. That is, all tables from the census will account for the whole population.
Recognising the increased difficulties in conducting censuses in the UK, particularly those experienced in the 1991 census, the Office for National Statistics (ONS) and its fellow census offices in Scotland (NRS) and Northern Ireland (NISRA), adopted a new approach to the census in 2001. Rather than relying on a single enumeration, the strategic approach to the census in 2001 was to carry out a traditional census followed by a sample re-enumeration (the Census Coverage Survey).
The One Number Census
By combining the results of the two operations and using statistical methods, ONS has been able to derive census estimates representing 100 per cent of the population, that is as if the census had counted everyone. This is called a One Number Census and is a major step forward from previous UK censuses where users of the census results have had to make judgements as to whether the estimated level of under-enumeration affected the use that could be made of the results. In previous censuses, two population counts were published; one raw unadjusted census count, and a subsequent count including the estimated under-enumeration which formed the base for annual population estimates.
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. Thus census response in 1991, defined as the proportion of the population counted on returned census forms, was 96 per cent in England and Wales.
In 2001, coverage of the census in England and Wales was 100 per cent. Total overall response was 98 per cent. Using the same definition as for 1991 it is estimated that 98 per cent of the population in England and Wales was covered in 2001, the same as for 1991. This includes some 4 per cent of the population estimated to be resident in households identified by enumerators but from whom no completed census form was returned. Census response in 2001 for England and Wales is therefore estimated to be 94 per cent. Corresponding census response and coverage rates for 1991 and 2001 for England and Wales separately are shown below, together with figures for England and Wales as a whole.
|Components of UK Census response and coverage rates for 1991 and 2001 - England and Wales|
|England||Wales||England & Wales|
People on returned forms:
Census Response Rate
Other people in identified households
Total overall response
People not included on returned forms and people in wholly missed households
Proportion of population covered in census results:
Census Coverage Rate
|Note: The 1991 rates shown are subject to slight change, but this does not affect the conclusions to be drawn from this analysis.|