The AHS-2 cohort consists of adult members of Seventhday Adventist churches geographically spread throughout the USA and Canada. Over 25 000 cohort members are blacks; the remaining subjects are of other races, mostly white. Recruitment and selection methods of the parent cohort have been described previously(5 (link)). Participants in the calibration study were randomly selected from the parent cohort by church, and then subject-within-church. Because of the special focus on black Adventists as a minority group in the AHS-2, the calibration study was designed such that there would be approximately equal numbers of blacks and whites. No significant differences in the distribution of gender, age, education or vegetarian status between the calibration sub-study sample (n 1011) and the AHS-2 cohort (n 96 592) were observed. When stratifying by ethnicity, only the distributions of mean age and age categories in blacks were nominally significantly different (P = 0·0018 and 0·0032, respectively) between the parent cohort and the calibration study sample (Table 1). This is easily consistent with chance given the multiple testing involved.
The duration of the calibration study was 9–12 months for any one subject (see Fig. 1). We obtained a set of three variably timed 24 h dietary recalls (one Saturday, one Sunday and one weekday intake) during the first two months, which then was duplicated approximately six months later. The aim was to obtain two sets of recalls (a total of six 24 h recalls) from each subject. During the 6-month interval, subjects completed a self-administered food FFQ that was identical to the baseline FFQ and attended a local study clinic. The study was approved by the institutional review board of Loma Linda University, and all subjects gave written informed consent.