2001 Samples of Anonymised Records
Introduction
Following extensive consultation with users a request was submitted to ONS in September 2001 for three datasets: a 3 per cent Individual SAR, a 1 per cent Household SAR and a 5 per cent Small Area Microdata file (SAM). The latter was particularly requested by geographers who were concerned with obtaining more geographical detail at the expense of individual information. The full details of the justification for the request and the specifications can be downloaded here.
In brief the request sought to:
- reduce the population threshold for SAR areas in the Individual SAR from 120k to 90k
- increase the sample size for the Individual SAR from 2 per cent to 3 per cent
- include additional detail in some of the variables, for example ethnic group, family type and professional qualifications, to reflect changes in the information collected in 2001
- add extra variables (to reflect the new questions asked in the 2001 Census).
However, increased concerns about confidentiality of microdata has resulted in the Individual and Household files being significantly less detailed than the original request and also less detailed than the 1991 SARs. In response to this ONS have established Controlled Access Microdata Samples (CAMS) which are only accessible within a safe setting in the statistical offices of the UK.
Both concern over under-enumeration and the importance of obtaining accurate population estimates and also concern over confidentiality have had marked influences on the 2001 census. The former has resulted in the ‘One Number Census’ and the latter has influenced both the outputs available and also their timing. Both are discussed in the sections below.
The One Number Census
The 2001 Census aimed to maximise coverage and to make an accurate estimate
of the people missed. The 1991 Census was thought to have had a substantially
larger under-count than in previous censuses with about 2 per cent of
the population of GB missed entirely and a further 1.6 percent for whom
records were imputed.
The One Number Census was designed to produce figures from the 2001 Census that are adjusted for under-enumeration and which are consistent across all forms of output and at the smallest geographical area. The term ‘One Number Census’ indicates a departure from the 1991 Census where preliminary figures from the census count were published and then later figures, adjusted for under-enumeration, were published. The One Number Census approach makes all adjustments as part of the census processing. Thus the One Number Census results in a database of the complete population for the UK from which all census outputs – including the SARs - are drawn.
The key stages of the ONC can be summarised as follows:
a) A Census Coverage Survey (CCS), undertaken independently of the Census,
was designed to establish the coverage of the 2001 Census. For the CCS,
the UK was divided into one hundred and twelve areas, each with a population
of about 500,000. These areas are known as design groups and are made
up of whole LADs or groups of smaller LADs. The CCS took place in all
of these design groups.
b) The CCS records are matched with those from the Census using a combination
of automated and clerical matching.
c) Populations for each design group, by age and sex, are estimated using
a combination of standard estimation techniques.
d) Small area estimation techniques are used to estimate Local Authority
District populations by age and sex.
e) Households and individuals estimated to be missed by the Census are
imputed to produce a fully adjusted Census database.
f) All ONC population estimates are quality assured using demographic
analysis and aggregate level administrative data.
More details on the One Number Census is available on the ONS website.
Last updated 4 January 2012
