The cumulative incidence of use was estimated in the conventional fashion, as the proportion of respondents who ever had a given disorder up to their age at interview. AOO distributions and projected lifetime risk at given ages were estimated using the two-part actuarial method implemented in SAS 8.2 [34]. The actuarial method differs from the more familiar Kaplan-Meier [35] method in using a more accurate way of estimating the timing of onsets within a given year [36], although both methods assume constant conditional risk of onset at a given year of life across cohorts. Sociodemographic predictors of lifetime risk were examined using discrete-time survival analysis with person-year as the unit of analysis [37]. The predictors considered here were sex, cohort, and a sex-by-cohort interaction. Cohort was defined by age at interview. We also examined a categorical version of the cohort variable, which distinguished respondents who were in the age ranges 18–29, 30–44, 45–59, and ≥60 y at interview. The sex-by-cohort interaction was examined to determine if the well-known gender difference in drug use has become smaller in recent years in
