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Frequently Asked Questions: 2011 Census Unrounded Population and Household Estimates for Wales This product is designated as National Statistics

Released: 24 September 2012 Download PDF


On 16 July 2012, the Office for National Statistics (ONS) released the first results from the 2011 Census for England and Wales – the ‘Population and household estimates for Wales’.

This included information for unitary authorities about the usually resident population of Wales by 5-year age and sex, and also provided information on topics such as the number of households and the number of short-term residents.

On 24 September, the day before publication of the mid-2011 population estimates (which are based on unrounded 2011 Census estimates), ONS has released the unrounded figures for those included in the 16 July census release.


Frequently asked questions

Are these unrounded estimates a revision to those published in July 2012?

No. The estimates in this unrounded release are a planned update to those published 16 July 2012 – they are the unrounded estimates made available now that the final few stages of data processing are complete. The estimates released in July 2012 were published to provide users with access to the first release of census data and also allowed for them to be used in the preparation of the mid-year population estimates. At the time, ONS explained the need to round these estimates, and began plans to release the unrounded estimates in time for the publication of the mid-2011 population estimates. This release was announced in the ONS release calendar and on the UKSA publication hub in late July.

The differences between the estimates published 16 July and those published 24 September are minimal. The differences for every age/sex estimate at local authority level are always smaller than 100. In fact only three per cent of these estimates are different by more than 50.

Why are the unrounded estimates being published now?

At the time the rounded estimates were published on 16 July a few of the final stages of data processing were yet to be completed, including statistical disclosure control which ensures that no personal information is disclosed and that no individual can be identified in census results.  These final stages of processing are now complete.

Why were the estimates published in July rounded to 100?

The estimates that were published 16 July were rounded to the nearest 100 because at that time a few of the later stages of processing had not yet been completed. Rounding protected against any changes that might arise from later stages of data processing, such as statistical disclosure control.

If I apply rounding to these unrounded estimates will I notice any differences in the figures compared to those published 16 July?

The differences between the estimates published 16 July and those published 24 September are minimal. The differences for every age/sex estimate at local authority level are always smaller than 100. In fact only three per cent of these estimates are different by more than 50. As the estimates in the July release were independently rounded, no local authority total is different by more than 100.

How do these estimates relate to or compare with the mid-2011 population estimates?

The mid-2011 population estimates are based on the unrounded 2011 Census estimates and will be published on the Office for National Statistics (ONS) website 25 September 2012.

The census estimates refer to 27 March 2011 whereas the mid-year population estimates refer to 30 June 2011. An estimate of population change has been made between these two dates to incorporate births, deaths and net migration that have occurred in the intervening period.

The two sets of estimates are also based on slightly different definitions of where individuals should be considered residents, and therefore where they are counted. For example, the census included armed forces personnel at their permanent family home but the mid-2011 estimates include armed forces where they are stationed.

Further information about the differences in definitions used is available with the mid-2011 population estimates release that will be published 25 September.

Why are the census statistics called estimates this time?


As with any census there will always be a small percentage of people who will have been missed and an even smaller proportion who will have been counted twice (perhaps because they live in more than one home, for example, students). This is also true of censuses internationally.

The Office for National Statistics (ONS) has used statistical procedures to deal with this, including information from the Census Coverage Survey and other quality assurance measures. Where people have been missed, these measures enable us to estimate their numbers and characteristics and produce estimates of the whole population.

When will more results from the 2011 Census be published?

ONS will be producing further, more detailed information from the 2011 Census in phases.  The first of these phases will be between November 2012 and February 2013. This phase will include the first statistics for local areas. A further two main phases will include the release of more detailed results later in 2013.  Further information about the phasing of releases is available in the 2011 Census Output prospectus.

Why are more detailed results not available sooner?

Although the processing of the data that was collected is now complete, and the estimates of the population have been fully quality assured, there is still work left to do to produce detailed results.

The next main phase of statistics published will cover the full range of topics from the census - providing the most detail possible about each individual topic covered - for a range of areas down to the most local level, the Output Area.

A number of checks are also needed to further assure the quality of the final detailed results that will be published, and to ensure that the protection of personal information is guaranteed when detailed information is prepared for small local areas. Work is well underway to complete these final steps and begin to publish detailed results in phases from November 2012.

Background notes

  1. Details of the policy governing the release of new data are available by visiting or from the Media Relations Office email:

    These National Statistics are produced to high professional standards and released according to the arrangements approved by the UK Statistics Authority.

Supporting information


Output Area (OA)
For the 2011 Census, the smallest geographic unit for which outputs are published is the Output Area (OA). Output Areas are used as the building blocks that are aggregated to form all higher geographical areas for which statistics are produced, in accordance with the Geography Policy for National Statistics. These ‘best-fit’ estimates are therefore consistent and comparable with other National Statistics prepared using the geography policy.

Statistical Disclosure Control (SDC)
The confidentiality of personal census information is paramount, and to prevent the inadvertent disclosure of information about identifiable individuals a range of statistical disclosure protection measures are used. Firstly, records in the output database are swapped between different geographic areas. This swapping is targeted towards those households in small areas with unusual characteristics that may be identifiable. To offer further protection against revealing personal information, some limitations have also been placed on the amount of detail available in the published results, particularly in tables for small populations. There are also minimum thresholds applied for the numbers of persons and households that must be present in the smallest areas for which sets of results can be produced.

Usual resident
The main population base for outputs from the 2011 Census is the usual resident population as at census day (27 March 2011). Although the population base for enumeration included non-UK short-term residents, these are not included in the main outputs from the 2011 Census, but are analysed separately. All outputs, unless specified, are produced using only usual residents of the UK. For 2011 Census purposes, a usual resident of the UK is anyone who, on census day, was in the UK and had stayed or intended to stay in the UK for a period of 12 months or more, or had a permanent UK address and was outside the UK and intended to be outside the UK for less than 12 months.

Get all the tables for this publication in the data section of this publication .
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