Anonymized data on sex, month, and year of birth, Townsend deprivation index (an indicator of socioeconomic status), and geographic location are stored in the UK Biobank resource and were available for 8,761,869 of the 9,238,453 (94.8%) individuals sent an invitation letter, allowing us to compare the distributions of these characteristics between nonparticipating invitees and participants. The distributions of a range of sociodemographic, physical, lifestyle, and health-related characteristics of the UK Biobank cohort were also compared with publicly available summary data from nationally representative population-based surveys and the UK Census. We selected summary survey data that matched the UK Biobank cohort as closely as possible with regard to population demographic factors (i.e., both sexes and ages 40–69 years) and the period of data collection (2006–2010). Where certain characteristics from the national survey summary data were only available in prespecified aggregated age and sex subgroups, UK Biobank data were stratified into similar groups for comparative purposes. Formal statistical tests of the difference in characteristics between UK Biobank and national data were not performed because of the lack of variance measures required to test for differences between means, such as standard deviations, from the comparison populations.
