The large size of the cohort was based on statistical power calculations for nested case-control studies [7], showing that 5,000–10,000 cases of any particular condition would be required for the reliable detection of odds ratios (ORs) for the main effects of different exposures of 1.3–1.5 (the upper end of the range reported from genome-wide association studies of various conditions [8]), and around 20,000 cases for detection of interactions with ORs of at least 2.0. To observe such large numbers of cases of particular diseases within a reasonable follow-up period, prospective cohorts need very large numbers of participants. Projected numbers of cases of a range of common conditions expected to occur among 500,000 UK Biobank participants during 20 years of follow-up (Table 1) suggest that reliable assessment of the main determinants of most of these conditions (and others that are similarly common) should be possible during the current decade [6,9]. The age range for inclusion of 40–69 years represented a pragmatic compromise between participants being old enough for there to be sufficient incident health outcomes during the early years of follow-up and young enough for the initial assessment to occur before incipient disease had a material impact on exposures.
