Output areas

The ONS policy for England and Wales output geography is to maintain stability for 2011 Census outputs. Some modification of the output areas (OAs) and super output areas (SOAs) created from 2001 Census data will be required to take account of any significant changes since the last census. The total change applied to the output area hierarchy, however, is estimated to be no more than 5 per cent overall.

If change in OAs and SOAs does occur, it will take place where:

  • significant population change has occurred since 2001

  • OAs need to be aligned to local authorities that have changed their boundary between 2003 and 2011

  • some instances of 2001 output areas were considered to lack social homogeneity when they were created from 2001 Census.

  • there is a need to align the output area boundaries at the England/Scotland border, as should have happened when they were created

Thresholds used in the modification of OAs/SOAs

On behalf of the Office for National Statistics (ONS), the University of Southampton developed an algorithm to automatically modify output areas and super output areas that fall into one or more of the categories above. Where an OA or SOA has breached a specified upper population threshold (therefore have become too large), it will be split. Where an OA or SOA has breached a specified lower population threshold (therefore have become too small and potentially disclosive), they will be merged with an adjacent OA or SOA. Using splits and mergers of the existing output areas and super output area hierarchy, rather than total redesign, allows direct links between 2001 and 2011 OAs or SOAs and better comparison between 2001 and 2011 Census data.

Merges will be applied where:

  • output area population falls below 100 or 40 households

  • lower super output area population falls below 1,000 or 400 households

  • middle layer super output area population falls below 5,000 or 2,000 households

Splits will be applied where possible when:

  • an output area population exceeds 625 persons or 250 households

  • lower super output area population exceeds 3,000 persons or 1,200 households

  • middle layer super output area population exceeds 15,000 persons or 6,000 households

Although these upper population thresholds will apply, there may be exceptional circumstances where splitting an OA or SOA is not possible. Where splits occur, postcode building blocks will used to create two or more new OAs, constrained to the boundary of the original OA from which they were created. The use of postcode building blocks is consistent with the methodology applied in 2001, and allows the production of postcode to output area look-ups.

Workplace zones

Output areas were created for the analysis of residential statistics, using residential population and household data. As a result, they are of limited use for workplace statistics, as there is no consistency in the number of workers or businesses contained within an output area.

For 2011 Census, a new geography of workplace zones will be created as a more suitable geography for disseminating workplace statistics.

Workplace zones will be created by splitting and merging the 2011 output areas to produce a workplace geography that contains consistent numbers of workers, and nests within the existing output area hierarchy. Workplace zones will also be constrained to middle layer super output areas (MSOAs) to provide consistency between the OA and workplace zone geographies, and to allow comparison of the 2001 and 2011 Census workplace outputs at the MSOA level.

Workplace zones will be produced for England, Wales and Northern Ireland, but not for Scotland.

The research on creating algorithms for maintenance of the output areas and creation of the workplace zones has been carried out by the University of Southampton, in collaboration with ONS.

Cockings S, Harfoot D, Martin D, Hornby D, 2011. Maintaining existing zoning systems using automated zone design techniques: methods for creating the 2011 Census output geographies for England and Wales, Environment and Planning A, 43(10), 2399-2418. doi:10.1068/a43601

Martin D, Cockings S, Harfoot A. In Press. Development of a Geographical Framework for Census Workplace Data, Journal of the Royal Statistical Society, Series A.