OAs have been used for data collection and output of Census data since the 2001 Census. The OA is the lowest geographical level at which census estimates are provided. Output areas were introduced in Scotland at the 1981 Census and in all the countries of the UK at the 2001 Census.
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 (homogeneity was not used as a factor in Scotland).
Urban/rural mixes were avoided where possible; 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 were 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 split between two or more OAs.
The minimum OA size was 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.
The OAs for Northern Ireland had the same minimum size, as for England and Wales, but were based on postcodes as at January 2000. The OAs fit within the 2001 electoral ward boundaries.
In Scotland OAs were based on postcodes as at December 2000 and related to 2001 wards. However, the OAs did not necessarily fit inside ward boundaries where confidentiality issues made it more appropriate to straddle boundaries. The minimum OA size was 20 resident households and 50 resident people, but the target size was 50 households.
2011 Output Areas
England and Wales:
Maintaining stability as far as possible was key for the 2011 Census. Some modification of the previous Output Areas and Super Output Areas has taken place where a significant need has occurred since 2001 (see 'Modification of output areas' below).
The total of 2011 OAs is 171,372 for England and 10,036 for Wales. There are now 181,408 OAs, 34,753 LSOAs and 7,201 MSOAs in England and Wales. This means that 2.6 per cent of 2001 OAs have been changed as a result of the 2011 Census, along with 2.5 per cent of LSOAs and 2.1 per cent of MSOAs.
Significant points of interest for the 2011 Census are that output areas and super output areas align to local authority boundaries, including those that changed between 2003 and 2011, and also align at the border between Scotland and England. 161 OAs and SOAs were modified because they were considered unsuitable for reporting statistics. The average population in an OA has increased from 297 in 2001 to 309 in 2011.
Boundaries are available clipped to the coastline, for mapping as well as to the extent of the realm, for geographic information systems and analysis. All output areas and super output areas have unique nine character codes, in line with all statistical geographies provided by ONS.
An upper layer super output area (USOA) will not be created for England as part of the 2011 Census OA hierarchy. Data Unit (Wales) will create a set of 2011 USOA for Wales.
Boundaries are freely available under the terms of the Open Government Licence.
2011 Output Areas for Scotland will be released in Spring, 2013.
In Northern Ireland the 2001 Output Areas have been merged to produce new, 2011 'Small Areas' that fit within Super Output Areas. There will be c4500 small areas.
Modification of Output Areas in England and Wales
Changes in OA and SOA boundaries for the 2011 Census have taken place when:
significant population change has occurred since the 2001 Census.
local authority boundaries have changed between 2003 and 2011.
OA boundaries have been realigned to the England/Scotland border, as should have happened for 2001.
areas have been independently assessed as lacking social homogeneity when they were created for 2001.
Redesigned Output Areas and Super Output Areas:
do not align to ward and parish boundaries that have changed since 2003.
do not necessarily align to real-world features.
contain more than 100 persons and 40 households, even if they contain one or more communal establishments.
A number of geography reference data products for electronic download, alongside the relevant stage of 2011 Census statistics release are available. These include:
digital boundaries of the modified output areas, lower layer super output areas, middle layer super output areas and workplace zones.
lookups between the 2001 and 2011 OAs, where they have changed.
lookups between OAs, postcodes and a number of census output geographies.
Thresholds used in modification (England and Wales)
ONS runs a process to automatically modify those output areas and super output areas whose 2011 Census populations have significantly grown or declined since 2001. If output areas breached a specified upper population threshold (their populations became too large), they were split into two or more OAs using postcodes as building blocks.
Splits were applied where:
an output area population exceeded 625 people or 250 households.
a lower layer super output area population exceeded 3,000 people or 1,200 households.
a middle layer super output area population exceeded 15,000 people or 6,000 households.
There may be exceptions where an area that was above the population threshold could not be split. Where splits occur, building blocks of postcodes were used to create two or more new OAs (constrained to the boundary of the original OAs from which they were created). The use of building blocks (postcodes) is consistent with the methodology applied in 2001, and enabled production of postcode to OA lookups.
Where output areas or super output areas breached a specified lower population threshold (their population became too small and is therefore potentially disclosive) they were merged with an adjacent OA or SOA.
Merges were applied where:
an output area population fell below 100 people or 40 households.
a lower layer super output area population fell below 1,000 people or 400 households.
a middle layer super output area population fell below 5,000 people or 2,000 households.
Using splits and merges of the existing OA and SOA hierarchy, rather than a total redesign, allows better linkage and comparison between statistical outputs for the 2001 Census and 2011 Census.