Second, to examine reciprocal and time-varying relationships between substance involvement and suicidal thoughts and behaviors, we adopted a survival analytic approach. Because onset of substance use and SI and SA temporally clustered around age 13–16 years, a time-varying approach was adopted whereby an independent variable was considered a “predictor” only if it had an onset age prior to the age of onset of the dependent variable. For instance, in order for SI to be a predictor for onset of alcohol use, age at onset of SI was required to be less than the age at onset of alcohol use (Supplemental Table S1 provides proportions of individuals reporting substance involvement prior to SI/SA and vice versa). There were multiple reports of age of onset; for the current analyses, the report that was closest in age to the age at interview was taken, assuming that it would minimize recall bias. All covariates were also coded to precede the onset of the dependent measure as well.
