The 2001 Census Output Areas are designed specifically for statistical purposes. They are based on data from the 2001 Census and were built from postcode units. Output Areas are used not only for Census output but also as the basis of Super Output Areas which have been introduced as stable and consistently sized areas for Neighbourhood Statistics.
This contrasts with Censuses from 1961 to 1991 when small area statistics were aggregated and presented for areas drawn up for the conduct of the Census in the field - Enumeration Districts (EDs) - each of which was the workload for an enumerator. The size and character of each ED was determined for operational purposes. It was also convenient to use the ED codes in processing and as the smallest output 'building brick', but variability made them less than ideal for statistical purposes.
There are some 175,434 Output Areas in England and Wales. Some 37.5% lie between 120 and 129 households, whilst 79.6 per cent lie between 110 and 139 households. Some 5 per cent lie between 40 households - the confidentiality threshold - and 99 households, and many of these are single parishes (or Communities in Wales). The distribution of size by households has a single sharp peak, showing a consistency in size compared with the number of peaks which appeared in the distribution of 1991 Census Enumeration Districts by size.
The number of Output Areas in each authority, together with information on the operative dates for the district, ward and parish boundaries in each authority is available as an Excel file.
The authorities are arranged on the Excel spreadsheet as viewed initially in the standard order for presentation of statistics: Greater London; Metropolitan areas; Unitary Authorities in England (Government Office Region order); Unitary Authorities in Wales (North to South geographical order); Counties in England (alphabetical order). Where no operative date is shown for an authority, boundaries used in the creation of Output Areas are as at Census Day (29.04.01).
Super Output Areas
Larger standard building bricks - known as Super Output Areas - for use in official statistics have been produced outside the 2001 Census programme by grouping Output Areas by automated zoning methods. Census Key Statistics and some Census Area Statistics (CAS) tables are available for Super Output Areas.
More information about Output Areas and Super Output Areas is available from ONS Geography.
Harmonisation of Boundaries
The division of England and Wales into Output Areas was completed in late January 2003. The release of the Output Area boundaries was later than planned, although in time for preparations before release of the local statistics. The main change in the timing of the production of Output Areas was to allow compliance with a new National Statistics policy to harmonize output to boundaries as they exist at the end of each calendar year. The Output Area boundaries were therefore produced to fit the ward and parish/community boundaries legally in place on 31 December 2002, with production continuing into January 2003. This also met the very widesperead customer demand for output for up to date areas.
The availability of the Output Area boundaries was also delayed by an additional month to allow a limited number of changes and corrections after quality assurance. Perhaps the most significant is that the 'extent of the realm', which includes areas of sea and which was used in the production of Output Areas, has been retained as the containing outer boundary of the Output Areas in preference to mean high water mark. This avoids some splitting of coastal Output Areas into two or more parts, but area measurements for coastal Output Areas are for the part above mean high water mark to avoid distortion in density calculations.