Exact Census counts were produced only for a single geography - Output Areas, parishes/Communities and wards, or areas built exactly from them, as at 31.12.2002. Other geographies, with the exception of National Parks, are built from a 'best fit' of these component building bricks. This is to remove a risk that information about identifiable people or households might be disclosed by the differencing of standard sets of statistics for overlapping areas.
The 'best fitting' of a divided Output Area or ward allocates all records to the part in which the most population fell, as indicated by the co-ordinate references of the records. This process can be carried out only with the use of the confidential data held by ONS.
Output for geographies built from best fitting Output Areas is in general limited to the Census Area Statistics (CAS) released at Output Area level. Further consideration may be given to the release of the fuller Standard Tables which are likely to be needed for certain areas, often with large populations, such as health areas (which may not always build from wards), and large aggregates of Output Areas such as 'rural' England.
Special arrangements were made for wards (electoral divisions in Wales), parishes/Communities, and postcode sectors which had populations too small for the release of the usual output for such areas. More information, and lists of affected areas, is available in 'Amalgamation of geographic areas'.