Census 2001 Key findings and actions from the One Number Census Quality Assurance process 1 Contents Page 1 Introduction 3 2 Overview and summary 3 2.1 The One Number Census process 3 2.2 Imputation summary 4 2.3 Overview of the ONC Quality Assurance process 5 3 Key findings from the Quality Assurance process 5 3.1 Detailed findings identified from the Quality Assurance process 5 3.1.1 1991 Under-enumeration adjustments 6 3.1.2 London investigation 6 3.1.3 Babies in areas with large ethnic populations 18 3.1.4 Post stratification 19 4 Investigations resulting in adjustments 19 4.1 Contingency measures 19 4.1.1 Collapsing strata 20 4.1.2 Borrowing strength 20 4.1.3 Borrowing strength for babies 22 4.2 Population subgroup analyses 22 4.2.1 Full time students 23 4.2.2 Home Armed Forces personnel 24 4.2.3 FAF personnel 25 4.2.4 Prisoners 25 5 Dependency 26 6 Response rates 27 7 Further analysis and findings resulting from the quality assurance process 30 7.1 Comparisons with administrative and demographic data used in the Quality Assurance Process 30 7.1.1 Comparison with administrative sources 30 7.1.2 Comparison with MYEs 31 7.1.3 Sex ratios 35 8 Conclusion 36 References 37 Annex A: Glossary of acronyms 38 Annex B: Household and person imputation analysis 39 Annex C: Matrix of key themes and findings 42 Annex D: HtC levels and age-sex groups collapsed by Design Group 57 Annex F: LADs that received student halls of residence adjustments 70 Annex G: Defence communal establishments that received an adjustment for undercount among the armed forces 71 2 Key findings and actions from the One Number Census Quality Assurance process 1 Introduction Finally there is a section that outlines the 1.1 The aim of this report is to provide 2001 dependency adjustment made to the ONC Census users with a detailed insight into the key estimates. Please note that there are both pre and actions and findings raised during the Quality post-dependency figures in this report. Assurance review of the One Number Census (ONC) 2001 population estimates. It aims to 2 Overview and summary highlight how the quality assurance process 2.1 The One Number Census process adhered to the strategy agreed with census users 2.1.1 Following evaluation of the 1991 Census in December 2000 whilst further highlighting a number of new initiatives were introduced to the rigorous additional analyses that were maximise coverage in the 2001 Census. These undertaken throughout the quality assurance included: process and adjustments that resulted from these analyses. • the use of just one population base in order to avoid the coverage issues that 1.2 The report provides a brief overview of were experienced from the 1991 Census the ONC project and a summary of the types population definitions of people and households missed by the 2001 • encouraging people to post back their Census but imputed as a result of the ONC census forms to enable census fieldstaff methodology and imputation system. A brief to focus on those most likely to have description of the quality assurance process difficulty filling in their forms; that underpins the population estimates arising from the 2001 Census is also provided. Further • smaller workloads for staff in the more detail on the ONC process is provided in difficult areas; the “One Number Census methodology and • redesigned and carefully tested forms and Quality Assurance process” 1 report. This is a questions; supplementary paper that provides Census users • a community liaison programme with a comprehensive overview and description including translation of census material of the stages of the ONC process with further into 26 languages; and detail on the quality assurance methodology and description of the steps taken to quality assure • a focused programme of awareness the 2001 ONC estimates. raising and publicity. 1.3 Within this report the actions and key 2.1.2 However, as mentioned above, it was findings resulting from the quality assurance recognised that 100 per cent response would not process are explored in detail to illustrate the be achieved. A pattern of falling census response sorts of issues investigated and analysed during rates is evident in many other census-taking this process. Further evidence based on work countries. In addition the 1991 survey conducted done subsequent to the quality assurance process to estimate those that were missed was not of is provided to confirm the patterns and issues a large enough scale to identify fully the extent raised throughout the quality assurance process and distribution of the under-enumeration. As and to indicate where further investigation was a result, it was necessary to base the national needed. This work was carried out after the population estimates for 1991 on demographic ONC estimates were available for each Local estimates. This process was only valid at a high Authority area. level of aggregation and the census counts had to be adjusted down to local authority level to 1.4 Further information is provided on make them add up to the national total based on 2 the results of the 2001 Census in terms of the demographic estimate . the response rates to look at the pattern of underenumeration measured by the ONC. 2.1.3 In consultation, users were clear that for 2001 they wanted a fully adjusted set of counts covering 100 per cent of the population. The 3 www.statistics.gov.uk ONC Quality Assurance process www.statistics.gov.uk ONC Quality Assurance process ONC project was set up to meet this need. By • all population estimates were carefully conducting a redesigned and much larger post- quality assured using demographic enumeration survey (the CCS) and combining analysis and comparison with aggregate the results of both the census and CCS in what level administrative data. is known as a dual system approach, the aim of the project was to estimate and adjust the Further detail on the ONC methodology and census database for under-enumeration so that processes is outlined in the “ONC methodology 1 all statistics add up to ‘One Number’ 3 .The and Quality Assurance process” report. estimation strategy is described in ONS (2001)4. It also aimed to ensure that robust results could 2.2 Imputation summary be obtained for each local authority area. Central 2.2.1 Despite every effort, it was always accepted to the consultation was the acceptance that the that the 2001 Census would not enumerate 100 census counts would have a confidence interval per cent of the population. Therefore, Census associated with them. 2001 was designed from the outset to take full account of this. The ONC project aimed 2.1.4 The ONC methodologies were researched to integrate the 2001 Census counts with the and developed over a number of years by a estimated level of underenumeration in the joint team of statisticians from the ONS and Census. the University of Southampton. The work was overseen by a Steering Committee that included 2.2.2 The ONC process resulted in 3,197,058 experts from central government, statistical people being imputed (6.1 per cent of the total agencies overseas, other academic institutions resident population) in England and Wales. Of and local government. In addition, census these: users were directly consulted at several stages • 53.1 per cent were males and 46.9 per in the methodological development process cent were females; through census user group meetings and special workshops. It is important to emphasise that this • 24.0 per cent were aged under 15, 34.4 methodology has had the most peer reviews and per cent were aged 20 – 24, and only 6.3 user consultations of all National Statistics - an per cent were aged 65+; example are the sixty publicly available research • 76.8 per cent were white and 10.9 per cent papers representing over five years of work on were Asian or Asian British comprised of the National Statistics website. A description of Indian, Pakistani, Bangladeshi or other the risk management process for the project is Asian ethnic groups; found in Holt et al (2001)3. • 64.7 per cent were single (never married); 2.1.5 The ONC process involved a number of • 43.8 per cent were ‘working’; and stages: • 30.4 per were imputed into households • a CCS was designed and conducted within terraced dwellings. independently of the census during May/ June 2001; The ONC process resulted in 1,280,999 households being imputed for England and • records from the CCS were matched to Wales. Of these: those from the 2001 census; • 49.2 per cent were one person households • populations of the sample areas were and 22.8 per cent were two person estimated from the results of the households, highlighting that the 2001 matching using dual system estimation Census was more likely to miss those in techniques which enabled an estimate of households of these sizes; and those persons missed by both the census and the CCS to be made; • 27.6 per cent of imputed households were purpose built blocks of flats or tenements. • populations for each local authority by age and sex were then estimated using a Further detail on the imputation rates for a combination of standard regression and number of variables based on information small area estimation techniques; collected in the 2001 Census is outlined in • households and persons estimated to Annex B. have been missed by the census were then imputed to produce a fully adjusted 2.2.3 In order to have confidence in these census database; and finally figures, a stringent quality assurance process 4 5 www.statistics.gov.uk ONC Quality Assurance process www.statistics.gov.uk ONC Quality Assurance process was undertaken.