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Super Output Areas (SOAs)

Super Output Areas (SOAs) are a geography designed to improve the reporting of small area statistics.

They were introduced initially for use on the Neighbourhood Statistics (NeSS) website, but it was intended that they would eventually become the standard across National Statistics.

Background

Until now the standard unit for presenting local statistical information had been the electoral ward/division. This has drawbacks though:

  • Electoral wards/divisions vary greatly in size, from fewer than 100 residents to more than 30,000. This is not ideal for nationwide comparisons, and also means that data which can safely be released for larger wards may not be released for smaller wards due to disclosure requirements (that is, the need to protect the confidentiality of individuals).

  • Electoral wards/divisions are subject to regular boundary changes. This creates problems when trying to compare datasets from different time periods.

It was therefore decided to develop a range of areas that would be of consistent size and whose boundaries would not change.

These would be built from groups of 2001 Census Output Areas (OAs) and would be known as Super Output Areas (SOAs).

For England and Wales it was decided to create three layers of SOA, as described below.

The Lower Layer SOAs and Middle Layer SOAs which are now available - see foot of page for details.

In Scotland a similar set of areas to the Lower Layer SOAs was released in February 2004.

These areas are referred to as 'data zones' and are rather smaller in population size than their Lower Layer SOA counterparts in England and Wales.

There are 6,505 data zones.

Further information can be found on the Scottish Neighbourhood Statistics website.

In February 2005, the Scottish equivalents of Middle Layer SOAs were released. These are called Intermediate Geographies.

There are 1,235 Intermediate Geographies, which like the Data Zones are smaller in population size than their Middle Layer SOA counterparts in England and Wales, each with a minimum population of 2,500.

In 2006, two areas added names to their Intermediate Geographies.

Further information can be found on the Scottish Neighbourhood Statistics website.

In Northern Ireland, there is only one layer of SOAs Lower Layer SOAs. These were introduced in February 2005.

There are 890 Lower Layer SOAs, each with a population of between 1,300 and 2,800 and were created from whole or split wards (defined using Output Areas).

Further details of Northern Ireland Lower Layer SOAs can be found on the NISRA website.

The three layers of SOA

Disclosure requirements mean that some sets of data can be released for much smaller areas than others.

To support a range of potential requirements it was decided to create three layers of SOA:

Lower Layer

Minimum population 1,000; mean 1,500. Built from groups of OAs (typically five) and constrained by the boundaries of the Standard Table (ST) wards used for 2001 Census outputs.

Middle Layer

Minimum population 5,000; mean 7,200. Built from groups of Lower Layer SOAs and constrained by the 2003 local authority boundaries used for 2001 Census outputs.

Upper Layer

It has been decided, that after consultation there was not enough interest generated to create Upper Layer SOAs.

Creating the SOA boundaries

The 34,378 Lower Layer SOAs in England and Wales (32,482 in England, 1896 in Wales) were generated by a computer programme which merged OAs taking into account measures of population size, mutual proximity and social homogeneity.

The boundaries were released to the public in January 2004.

The 7,193 Middle Layer SOAs (6,780 in England, 413 in Wales) were generated via a two-stage process:

  • A draft set was generated by computer, in the same manner as the Lower Layer SOAs.

  • Local authorities and other local agencies were invited to propose changes to the draft boundaries in order to establish SOAs that better meet local needs.

The consultation began on 8 March 2004 and ended on 4 May 2004.

The feedback from the consultation was processed, and the final set of Middle Layer boundaries were released in August 2004.

Further details of the consultation are available on the NeSS website.

The Upper Layer SOAs, after a public consultation in 2010, it has been decided that there is not enough interest to create this layer of SOAs.

Update of the SOAs

Since the English and Welsh SOAs were first created in 2004, there has been a 'consultation' undertaken to decide how best to deal with changing population and household counts.

The decision taken has been to maintain the SOAs and keep them as defined in 2004.

In cases where the population and household counts have breached the required limits, the following will be done:

  • Lower Threshold breached - The SOA will be merged with an adjacent SOA, so that the counts fall within the allowed limits. This will be the case for both Lower Layer SOA and Middle Layer SOA.

  • Higher Threshold breached - The Lower Layer SOA will be split to Output Area and merged with adjacent Lower Layer SOAs, so that the counts fall within the allowed limits. The Middle Layer SOA will be split to Lower Layer SOA level and merged with adjacent Middle Layer SOAs, so that counts fall within allowed limits.

These changes above will affect no more than 5 per cent of the SOAs, a figure that falls within the ‘ONS Geography Policy’.

The above changes will be made using the 2011 Census data, and will not be completed until 2013.

In Scotland, there are plans to make minor changes to the Data Zones after the 2011 census population figures are known. These changes will be similar to those in England and Wales.

In Northern Ireland, it is planned that the Lower Layer SOAs will be redesigned after the 2011 Census. This will follow a large local Government reorganisation.

Lower Layer SOAs: Constitutions and Boundary Files

Lookup tables relating 2001 Census Output Areas (OAs) to Lower Layer SOAs and Middle Layer SOAs are available on the NeSS website.

A CD-ROM with the Lower Layer and Middle Layer SOA boundaries is available free of charge from ONS Geography.

SOAs: Frequently Asked Questions

For more information on SOAs, a set of frequently asked questions is available on the NeSS website.

Content from the Office for National Statistics.
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