All the ONC population estimates have been subject to rigorous quality assurance. The quality assurance process followed an agreed strategy which had been the subject of wide consultation with census users. The population of each local authority by age and sex were considered in a consistent and detailed manner - this involved comparison against diagnostic ranges derived from rolled-forward population estimates and aggregated administrative sources (such as Birth Registration and Pensions data). These ranges gave a plausible set of values with which we would, prior to the census, expect the ONC estimates to be comparable. Where the ONC estimates fell outside of the diagnostic ranges, extensive checks of the ONC results were undertaken with respect to sample sizes, outliers, etc and contingency action was taken if any issues were identified.

The quality assurance process included analysis for each local authority of a number of specific population subgroups known from 1991 to be prone to under-enumeration. These were full-time students, home armed forces, foreign armed forces and their dependants and prisoners. The estimates for these subgroups were compared with data from other official sources to determine whether the results were plausible. This was especially important for students and prisoners as the CCS did not cover large communal establishments. Adjustments were made where significant under-enumeration was identified for these groups and in total this resulted in 27,000 persons being added to the estimates for England and Wales as a whole.

All ONC estimates were discussed and signed-off by an expert panel at a series of weekly meetings over the course of several months. These meetings adopted a rigorous and consistent approach throughout, with the results by age and sex for each of the 376 local authorities in England and Wales being considered in detail. The quality assurance process was also subject to external observation.