The ATR approached 7,850 twins born between 1972 and 1979, of these 3,786 twins consented to participate, 1,971 declined and the reminder gave passive refusals (5 attempts/calls without reaching respondent) or did not respond. In addition, upon interview with the twins, 1,218 siblings were eligible for participation and 849 were recruited of which 424 siblings completed both the interview and questionnaire. Siblings were recruited by QIMR staff and at least 10–15 attempts were made to contact siblings before classifying them as passive refusals. Respondents were also contacted weekly, over a period of 3 months, regarding the questionnaire. On completion of both the interview and questionnaire, all respondents received a $50 gift card. Table 1 provides the distribution of twins and siblings, by sex and paired status, who participated in the interview and questionnaire components of the project.
