Compiling the address register

The address register was central to the whole operational design of the 2011 Census which used Royal Mail to deliver the questionnaires to households. Having an address register enabled ONS to pre-address and uniquely code each questionnaire before the operation and to deliver those questionnaires by post. Having each questionnaire individually coded enabled prompt receipting of both paper and online returns giving accurate management information on progress and enabling field staff to be targeted at non responding households. It also underpinned data processing and the outputs.

ONS looked at the potential sources: the Royal Mail, Ordnance Survey’s MasterMap Address Layer 2 and the National Land and Property Gazetteer. Ordnance Survey's product was used to produce address lists for the 2007 Census Test. ONS subsequently carried out further research and development work to compile a national address register which it was able to trial with three local authorities during the 2009 Census Rehearsal.

The national address register formed the backbone of the 2011 Census operation. It enabled Royal Mail to post questionnaires to around 25 million households in England and Wales, and supported questionnaire tracking which in turn decided which households would be targeted for follow-up.

Building the national address register

This was a complex challenge. While a large core of residential addresses would not have changed since 2001, there would still be many that were new, changed or extinct.

Work started in 2006. The national address lists developed by Royal Mail and local government provided a wealth of largely accurate address information for ONS to compare, check and build upon. Any discrepancies between these products were resolved with the help of local authority staff and ONS address checkers.

Each local authority was asked to check an average of around 500 addresses. The number of mis-matches ranged from a handful in rural communities to several thousand in city areas and large councils.

In spring and summer 2010, a team of 350 address checkers physically checked the addresses that gave most concern. The team looked at 15 per cent of the addresses that were considered to be unreliable. This involved around 3.6 million addresses in the postcode areas that had the most mismatches and/or had the most addresses in multi-occupancy, such as several flats in the same building.

About a quarter of the address checkers concentrated on checking the details of student halls of residence, prisons, armed forces accommodation, nursing/residential care homes and other communal establishments which are likely to have the most influence on the quality of census statistics. ONS also looked at how best to count people in caravan parks, gypsy camps and other temporary/transient groups.

The address register was continually updated and refined up to March 2011 to make sure the data was as complete and accurate as possible.