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. Census data can be aggregated to any level of spatial unit, but the two base geographies are Enumeration Districts (EDs) and Output Areas (OAs). For the 2001 Census both units are, for the first time, being used across the whole UK. However, their definitions vary slightly in each country according to the organisations carrying out the Census. Although the Census date was the same across 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).
Enumeration Districts (EDs)
These units were formerly used for both data collection and presentation. Although their boundaries are contiguous with those of administrative geographies their primary function has always been the collection process, with size and shape designed to equalise the workload of Census enumerators. In terms of statistical presentation though this is not ideal and Output Areas (see below) are now being used. However, EDs are still used for data collection. For the 2001 Census England and Wales had 116,895 EDs (the majority of which are different to their 1991 equivalents) with an average size close to 200 households (450 people). Scotland has 6987 EDs at an average size of 328 households (730 people) and Northern Ireland has 2591 EDs at an average size of 260 households (650 people). Some EDs are termed Special Enumeration Districts (SEDs): these are communal establishments with the capacity to house over 100 people (actual population is unimportant); for example, prisons, hospitals, nursing homes, halls of residence, large hotels and military bases.
Output Areas (OAs)
For the purposes of statistical presentation, these are the successors to EDs and are built up of unit postcode areas. Used in Scotland since 1981, the outputs of the 2001 Census will see their introduction in England, Wales and Northern Ireland. OAs are smaller than EDs so can provide a finer resolution of analysis, and in addition are designed to have approximately similar populations, tend towards a hexagonal shape and be as socially homogenous as possible (based on tenure of household as stated on the Census form - note though that homogeneity is not used as a factor in Scotland, and that in Northern Ireland religion may ultimately replace tenure as the criterion). The main constraints however are that OAs must be built up of contiguous unit postcodes and must have a minimum size of 40 resident households and 100 resident persons (20 resident households and 50 resident persons in Scotland), for the purpose of protecting data confidentiality. This definition of OAs means that their boundaries will only be determined when the Census data is available, but the process is largely automated.
In England and Wales OAs will fit parish, electoral ward/division and higher administrative boundaries exactly, meaning that if a unit postcode area straddles a boundary, it will be split. If a parish or ward/division is too small to protect confidentiality, it will form part of a larger OA. In the latest round of consultations the recommended size was 100-125 households (250 people) but this may be lower in some rural areas. If this size is adopted there will be approximately 200,000 OAs in England and Wales.
In Scotland OAs will fit into wards and other geographies where possible but there may be occasions when confidentiality issues make it more appropriate to straddle boundaries. The target size of Scottish OAs is 50 households (130 people), with around 35-38,000 OAs expected.
In Northern Ireland OAs will fit into electoral wards and will typically have 80-100 households (230 people). Approximately 7,500 are anticipated.
As well as their key role in the Census, OAs will ultimately also constitute the base unit for Neighbourhood Statistics.
Information on obtaining Census Geography listings.
This page last revised: Monday, 19 November 2001