Census Geography: Wales 2001

For the 2001 census in Wales a new census geography design method was adopted which created for the first time "Output Areas" (OAs) at the lowest level, specifically for the purpose of publishing census data. These were computer-generated and intended to be of uniform population size, take account of postcode and ward boundaries and to be as socially homogeneous as possible. The 9,769 Welsh OAs contain an average of 297 persons and 124 households. The minimum threshold for publication of census data was 100 persons and 40 households and no OAs were created to be smaller than this. Because no definitive boundaries exist for unit postcodes, these were automatically generated as part of the design process and the OA boundaries can therefore contain large "spikes" and other irregularities reflecting the computer-generated nature of the underlying postcode boundaries.

OAs nest within local authority electoral divisions as of 31 December 2003, which was used as the reference date for the 2001 census geography. Some postcodes will be split across OA boundaries but OAs are an exact fit to electoral divisions. Electoral divisions vary widely in population size, but tend to be of similar population sizes within a single local government area. The local government tier comprises unitary authorities. The codes for these different levels are combined to create a 10-character alphanumeric code. Area names are applied to all levels about the OA. There are no counties in the Welsh administrative geography and the characters 00 are used in this field which may contain a county code in England. This census geography coding scheme will be found within postcode directories, census datasets and the Neighbourhood Statistics website.

A particular complexity with the 2001 census is that different population thresholds were set for different sets of statistical outputs. More detailed statistical tabulations known as the "standard tables" had thresholds of 1000 persons and 400 households and were intended to be available at the electoral division level. As some electoral divisions are less than the required size, there are unfortunately different versions of wards in a few instances, with the result that the census area statistics (CAS) electoral divisions and standard tables (ST) electoral divisions with the same area code may not refer to exactly the same geographical area!

Digital boundary data are freely available for all units in the hierarchy.

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