in phase I, participants completed computer assisted interviews. The Royal Mail's small users' postcode address file was used to sample private households. Postcode sectors were stratified within each NHS region according to socioeconomic profile. These were visited to identify private households with at least one person aged 16–64 years. The Kish Grid method (Kish, 1965) was used to select systematically one person in each household. A total of 8886 adults completed the first phase computer assisted interview. Data on the second phase clinical interviews are not included in this study.