Coverage assessment and adjustment processes

Every effort was made to ensure everyone was counted in the 2011 Census, however no census is perfect and some people were inevitably missed. This under enumeration does not usually occur uniformly across all geographical areas or across other sub-groups of the population such as age and sex groups.

To fill this gap, ONS developed a coverage assessment and adjustment methodology which built on the 2001 ‘One Number Census’ (ONC) approach. The methodology involved the use of standard statistical techniques, similar to those used by many other countries, for measuring the level of undercount in a census and providing an assessment of the characteristics of individuals and households missed. The approach was researched and developed over a number of years in consultation with academics, statisticians, demographers and users of census data.

An independent review of the coverage assessment and adjustment methodology took place in 2011. The review team stated 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”.

Development of methodology

The main aim of this methodology was to estimate, and adjust the census data for the number of people and households not counted in the 2011 Census. A secondary objective was to identify and adjust for the number of people and households counted more than once, or counted in the wrong place, in the 2011 Census.

The proposed strategy and resulting methodology for 2011 Census coverage assessment and adjustment were proposed and developed in a series of communications:

• 2011 Census Coverage Assessment and Adjustment Strategy
  Published in Population Trends 127, spring 2007

• 2011 UK Census Coverage Assessment and Adjustment Methodology
  Published as Census Advisory Group paper (08)05, spring 2008

• How are census estimates produced? Coverage assessment and adjustment in the 2011 UK Census
  Short leaflet, 2008

• 2011 UK Census Coverage Assessment and Adjustment Methodology
  Summary guide, 2008

• 2011 UK Census Coverage Assessment and Adjustment Methodology
  Published in Population Trends 137, autumn 2009