Data quality assurance

The 2011 Census population estimates have been through a rigorous quality assurance (QA) process to ensure that users of census data have confidence in the quality and accuracy of the information.

The QA approach was built on the principles outlined in the 2011 Census Quality Strategy and researched and developed over a number of years in consultation with academics, statisticians, demographers and users of census data. It was subject to an independent review which concluded that “the methods give confidence that the resulting final census population estimates will be better than any other method and will be suitable for use in resource allocation and planning”.

Quality assurance processes began in March 2011, when management information and early census returns provided the first evidence of response patterns and continued throughout the census process through to the publication of the first outputs. Following the coverage assessment process, all the population estimates were quality assured using demographic analysis, survey data, qualitative information and administrative data to ensure the estimates were plausible. More information about the QA process, data sources and findings are available.

Development of the quality assurance approach

In 2009 ONS published the 2011 Census - Data Quality Assurance Strategy which set out the management systems and processes which were being put in place in the run-up to the 2011 Census. The strategy took account of international best practice and also built on the lessons learnt from the QA of the 2001 Census. The 2011 Census - Data Quality Assurance Strategy is available to download.

ONS worked closely with users to finalise the QA approach to ensure that users were able to contribute to the strategy and make the process transparent. This included carrying out a series of Quality Assurance Studies with 40 local authority users which were aimed at better understanding the sources to be used in the QA process and identifying locally held information which might also be of value. The final report from these studies, the Evaluation of the Census 2011 Quality Assurance Studies project, is available to download.

Early in 2011, all local authorities were asked to provide locally held sources which the project had identified as being potentially useful. To help users understand and build confidence in the QA process, a series of ‘Spotlight on Completing the Count’ events were held in late 2010. These seminars provided an accessible explanation of the Coverage Adjustment and Quality Assurance processes to over 120 users. Slides presented at this event are available to download, along with questions asked and answers provided at the events.

To give users further confidence that the quality assurance methods proposed were robust, the National Statistician asked Professor Ian Plewis from the University of Manchester to lead an independent review of methods for coverage assessment, coverage adjustment and quality assurance of the 2011 Census population estimates for England and Wales. The review team made 23 recommendations and ONS made an initial response to these on 16 March. Downloads of the recommendations and the papers ONS published in response are available via the 2011 Independent review link.

The quality assurance methodology was refined to take into account their recommendations and culminated in the publication of the 2011 Census – Methodology for Quality Assuring the Census Population Estimates, which is available to download. This document provided an overview of the plans for quality assurance of the census population estimates, including how the estimates would be agreed and signed off ahead of the publication of the first release in July 2012. On their recommendation, further details about the QA process were then published in the documents 'Guidance for moving from core to supplementary quality assurance', and 'Options for making national adjustments to LA level', both of which are available to download.

The UK Statistics Authority also carried a Special Assessment of the 2011 Census which designated 2011 Census statistics for England and Wales as National Statistics, in accordance with the Statistics and Registration Service Act 2007, signifying compliance with the Code of Practice for Official Statistics. The paper Overview of Administrative Comparator Data Used in 2011 Census Quality Assurance was published to meet their requirements and is available to download.