Distributions were examined for skewness and kurtosis; seven variables were found to deviate from normality (marijuana hits smoked per month, days since last marijuana use, years of regular drinking, days since last alcohol use, lifetime other drug use, cigarettes smoked per day, and FTND score) and were log transformed (Tabachnick and Fidell, 2007). Clusters showing significant group differences in FA or MD were examined in a repeated-measures analysis of covariance (ANCOVA) to control for familial and substance use variables that differed between groups. Regression analyses assessed the unique contributions of substance use and parental history of substance use disorder (SUD) on FA and MD within significant clusters; P-values of <0.004, corrected for multiple comparisons using Bonferroni adjustments, were considered significant. In follow-up exploratory analyses, mean FA and MD values in significant clusters were correlated with substance use variables in both groups using Pearson's r correlation coefficients (α=.05).
