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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.

Owing 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: homogeneity was not used as a factor in Scotland).

Urban/rural mixes were avoided where possible (that is, 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.

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 people 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 5,022 OAs in Northern Ireland.

In Scotland OAs are based on postcodes as at December 2000 and are related to 2001 wards.

Note: 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 people, but the target size was 50 households.

There are 42,604 OAs in Scotland.

Content from the Office for National Statistics.
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