It is possible that respondents in the present study (particularly those in Cohort II) may not have passed through the age period of risk for initiation of substance use. This is of minimal concern for alcohol, as 99.2% of the sample had used alcohol in their lifetime. However, this remains a possibility for tobacco and cannabis use. We therefore explored the potential impact of censoring. With regard to cannabis, in Cohort II, the cumulative initiation probability by age 24 (.51) was 84% of the initiation probability by age 36 (.61). In Cohort III, the initiation probability by age 24 (.61) was 90% of the initiation probability by age 36 (.68). For tobacco, in Cohort II, the cumulative initiation probability by age 24 (.87) was 98% of the initiation probability by age 36 (.89). In Cohort III, the initiation probability by age 24 (.85) was 98% of the initiation probability by age 36 (.87). The percentage of cannabis abstainers in both cohorts differed by only 8.2% (Cohort II: abstention rate=40.4%, Cohort III: rate=32.2%), and the percentage of tobacco abstainers differed by
