Weighted frequencies and their respective 95% confidence intervals (95% CI) were computed to characterize the sample. Estimated projections of the cumulative probability of transitioning from use to dependence within the first year of substance use onset, the first decade after substance use onset, and lifetime in the general population and by racial-ethnic group were obtained by the standard actuarial method (Machin et al., 2006) as implemented in PROC LIFETEST in SAS (version 9.1.3), (SAS Institute, Cary, N.C.). The log-rank test was used to determine whether survival curves differed statistically across substances and across racial-ethnic groups for each substance.
