Whether or not diversity of substances used increased over time was evaluated in Mplus using a Poisson-distributed linear growth model, with time points at 11, 14, and 17 years of age represented by 0, 1, and 2, respectively. The model indicated that there was an increase in substance use counts during adolescence for both males (slope=0.85) and females (slope=1.07) with a greater overall rate of growth for females than for males (z=3.53, p<0.001). The model in which the slope of substance use counts over time was allowed to be non-zero (AICmales=3573.4, AICfemales=3306.7) fit significantly better than when the model assumed no change over time (i.e., with the slope set to zero, AICmales=3958.3, AICfemales=3734.4; χ2males(1)=193.4, χ2females(1)=214.8, p<0.0001 for both). This model showed that, as expected, the total diversity of substances used significantly increased between ages 11 and 17 for both males and females.
