All analyses were done stratified by gender, given the substantial differences in base rates of substance use by gender (Keyes et al., 2008a), and evidence that racial/ethnic differences in trajectories of substance use differ by gender (e.g., (Caetano, 1984)). Age within measurement wave was included as a covariate in adjusted models, as was highest level of parental education, urban versus rural location (Gibbons et al., 2007), family use of public assistance, and household income in adolescence given evidence that racial/ethnic differences in substance use are partially accounted for by socio-economic status (Watt, 2008). All socio-economic indicators were significantly correlated. The strongest correlations were between parental education and family income (r=0.35) and use of public assistance and family income (r=0.47). By race, there were no average differences in baseline age (χ2=2.1, df=2, p=0.34); Black respondents on average had lower levels of parental education (χ2=21.9, df=2, p<0.01), lived in urban areas (χ2=44.8, df=2, p<0.01) were more likely to be in families that used public assistance (χ2=1099.6, df=1, p<0.01), and had lower household income in adolescence (χ2=598.8, df=1, p<0.01).
