Census Geography
The UK Census
The UK Census, undertaken every ten years, collects population and other statistics essential to those who have to plan and allocate resources. Major customers include departments of national and local government, and providers of services such as health and education. Although the Census occurs simultaneously in all parts of the UK, the responsible body in England and Wales is the Office for National Statistics (ONS), in Scotland the General Register Office for Scotland (GROS) and in Northern Ireland the Northern Ireland Statistics & Research Agency (NISRA). The most recent Census took place on 29 April 2001.
There are two forms of geography directly associated with the Census: Enumeration Districts (EDs) and Output Areas (OAs). EDs are now used for data collection only and OAs are the base unit for Census data releases. Note though that Census data can be output for any area required by amalgamating the appropriate OA data.
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Enumeration Districts (EDs)
EDs were formerly used for both data collection and output but their size and shape was primarily determined by the requirements of data collection. This was less ideal for data output so OAs (see below) have been introduced and EDs are now used for data collection only.
For the 2001 Census England and Wales had 116,895 EDs (the majority of which were different to their 1991 equivalents) with an average size close to 200 households (450 people). Scotland had 6987 EDs at an average size of 328 households (730 people) and Northern Ireland had 2591 EDs at an average size of 260 households (650 people). In addition there were the Special Enumeration Districts (SEDs), communal establishments with the capacity to house over 100 people (whether they actually had this number in residence was unimportant). Examples of SEDs include prisons, hospitals, nursing homes, halls of residence, large hotels and military bases.
1991 Census EDs fitted into the then administrative boundaries, but as EDs were no longer required for output, 2001 Census EDs sometimes straddled 2001 administrative boundaries.
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Output Areas (OAs)
Census data can be output for any area required, whether electoral wards/divisions, local authorities, health authorities or any other geographic unit. Such statistics were formerly aggregated from ED-level data but the new Output Areas (OAs) were introduced in Scotland at the 1981 Census and in England, Wales and Northern Ireland at the 2001 Census. Due to their smaller size, OAs allow for a finer resolution of data analysis.
2001 Census OAs were built from clusters of adjacent unit postcodes but as they reflected the characteristics of the actual Census data they could not be generated until after data processing. They were designed to have similar population sizes and be as socially homogenous as possible (based on tenure of household and dwelling type) - note though that homogeneity was not used as a factor in Scotland. Urban/rural mixes were avoided where possible (i.e. OAs preferably consisted entirely of urban postcodes or entirely of rural postcodes). They had approximately regular shapes and tended to be constrained by obvious boundaries such as major roads. The OAs were required to have a specified minimum size to ensure the confidentiality of data. |
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An example of 2001 Census OAs
(Locks Heath, Hampshire)
Scale approx. 1cm=50m. |
In England and Wales 2001 Census OAs are based on postcodes as at Census Day and fit within the boundaries of 2003 statistical wards (and parishes). If a postcode straddled an electoral ward/division (or parish) boundary, it was therefore split between two or more OAs. The minimum OA size is 40 resident households and 100 resident persons but the recommended size was rather larger at 125 households. These size thresholds meant that unusually small wards and parishes were incorporated into larger OAs. In total there are 175,434 OAs in England (165,665) and Wales (9,769).
In Northern Ireland OAs have the same minimum size but are based on postcodes as at January 2000. They also fit within 2001 electoral ward boundaries (there were in any case no differences between wards as at Census Day and the 2003 statistical ward set). There are 5022 OAs in Northern Ireland.
In Scotland OAs are based on postcodes as at December 2000 and are related to 2001 wards. Note though that OAs do not necessarily fit inside ward boundaries where confidentiality issues made it more appropriate to straddle. The minimum OA size is 20 resident households and 50 resident persons, but the target size was 50 households. There are 42,604 OAs in Scotland.
Note: due to the sheer numbers involved, and the capabilities of modern technology, OAs are generated by computer rather than by hand!
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Statistical Wards, CAS Wards and ST Wards
Census OAs in England and Wales have been designed to fit into 2003 statistical wards, which reflect administrative boundaries promulgated (laid down in statute) by 31 December 2002. Most 2001 Census outputs however use Census Area Statistics (CAS) wards - these are a subset of 2003 statistical wards, with particularly small wards merged to protect data confidentiality. Scotland meanwhile has a separate set of CAS wards based on amalgamations of OAs. In addition, Census standard tables use Standard Table (ST) wards, a subset of CAS wards. Further information on statistical, CAS and ST wards.
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Further Information/Census Geography Products
Geography for the 2001 Census in England and Wales (348Kb pdf file): Detailed paper by Professor David Martin of Southampton University, with a particular focus on OA design.
ONS offers a range of 2001 Census geography products for England and Wales including digital OA boundaries (vector format), look-up tables of OAs by postcode, and reference maps with OA boundaries overlaid on Ordnance Survey topographic maps. For more details contact us.
GROS's 2001 Census geography products for Scotland include digital OA boundaries and an OA:higher area look-up. GROS also provides more comprehensive information on Scottish census geography.
NISRA's 2001 Census geography products for Northern Ireland include a CD containing both digital OA boundaries and various look-up tables. More details here.
For resources relating to earlier censuses, please contact the relevant national body.
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This page last revised: Tuesday, 30 October 2007
geography