| Population resources for mid-1991 and mid-2001 -all by 2001 geography- |
Dissemination has been supported by Leverhulme award 20050099.
Online data for electoral wards
Methodology
Although the 1991 and 2001 Censuses in the UK have measured the principal variables to compare population over time and space, four standard but difficult problems of data harmonisation over time remain. These four problems are general to any country when comparing population estimates over time.
The enhancement of 1991 and 2001 Censuses consisted of the following:
1) Population definition
Who is included in the definition of population affects the population estimate published. In the UK, two differences between practice in the censuses of 1991 and 2001 are significant, the enumeration of students and population date.
Whilst the 2001
Census enumerated the whole population at the address of ‘usual residence’
including students at their term-time address, the 1991 Census enumerated
students at their vacation address. The transfer of students from
their vacation address to their term-time address in 1991 has a significant
impact on assessment of population change, by increasing the 1991 population
in areas with student campuses (often but not always within urban areas),
and decreasing other areas from which students leave to study elsewhere.
Because population
estimates are usually made for mid-year (30th June) rather than Census
day (a different day of April
in 1991 and 2001), an additional allowance for timing is necessary to
bring them both to the same population date. Although the net effect of
timing is small nationally, its impact locally can be significant.
2) Treatment of non-response
Since it is widely accepted that no census will count the whole population, adjustments are usually made for undercount and in some countries for compensating overcount. In England and Wales in 1991 and 2001 the treatment of non-response in 1991 and 2001 was substantially different.
In 1991 extra records for people in missed households were included in the census database and published output but a further 2% were estimated as missed from the census output. In 2001 the One Number Census (ONC) integrated a more complete estimate of non-response in the published census counts for all areas, with further non-response limited to about 0.5%. In both years, the non-response missed from census output was skewed towards young men, urban areas and minority ethnic groups. Plausible estimates based on evidence from post-enumeration surveys can been used to make allowances for this non-response.
3) Demographic classifications
While not resulting any change to the total count of population, changes
in recording and coding practices can render censuses incompatible, as
happened in England and Wales with ethnic identification and age group
categories. Whilst the 2001 Census recorded 16 ethnic group categories,
including four mixed categories, the 1991 Census output included 10 ethnic
group categories, with no mixed categories.
Analyses of ethnic group stability over time using the ONS Longitudinal Study (LS) data showed that reliable comparisons over time can be made for five groups: White, Indian, Pakistani, Bangladeshi and Chinese and less reliable comparisons for the Black Caribbean and Black African groups. The residual (‘Other’) ethnic groups of both 1991 and 2001 exhibit very low stability and, therefore, are not appropriate for comparisons. Classifications in which more groups are combined (such as ‘Black’) offer greater stability but less meaningful interpretation as they combine groups with very different demographic trajectories.
Although date of birth is captured during census fieldwork, published output uses age bands which are not compatible between censuses. For example age 85 and over in 1991 and 90 and over in 2001 for electoral ward and further discrepancies for smaller areas.
4) Harmonisation of geographical units
The geographical boundaries of most countries’ administrative units change over time, in ways that prevent calculation of
population change directly from output of successive censuses In England and Wales, small geographical units have been most affected by geographical boundary changes.
To achieve harmonisation of these geographical areas 1991 population estimates
for the smallest census areas are proportionally converted to 2001 Census
geographical units.
Meeting these
four challenges has resulted in a consistent time series for single years
of age, sex and ethnic group. The complete mid-1991 and mid-2001 population
estimates are fully consistent with the latest population estimates for
those years published by ONS, GRO(S) and NISRA.
In Scotland the time series is available only for Council Areas because of the lack of complete ethnic group output for smaller areas in both 1991 and 2001, but a time series of complete population estimates for 2001 Census CAS-Sector boundaries in Scotland by detailed age and sex is available, produced with Paul Norman of Leeds University. For Northern Ireland estimates for 1991 and 2001 mid-years are available for the 2001 CAS wards.
Documentation Documentation
- Sabater, A and Simpson, L (2007) Enhancing the population census: a time series for sub-national areas with age, sex and ethnic group dimensions in England and Wales, 1991-2001, CCSR Working Paper 2007-11, University of Manchester.
- Norman, P., Simpson, L. and Sabater, A. [Forthcoming, 2008] ‘Estimating with Confidence’ and hindsight: new UK small area population estimates for 1991. Population, Space and Place.