a general tendency to repeatedly use all three substances. By Wave 2, the prevalence of repeated-use of all three substances had increased to 15.45%. Similarly, the observed prevalence of non-repeated-use of all three substances at each wave (Wave 1, 75.29%; Wave 2, 32.77%) exceeded what was expected (Wave 1, 64.48%; Wave 2, 18.13%). These excess observations were balanced by corresponding deficits in the numbers using only one or two substances repeatedly or non-repeatedly. The overall χ2 (with four degrees of freedom) testing both two way and three way interactions was highly significant (Wave 1: χ2 = 754.62, p<0.001; Wave 2: χ2 = 470.42, p < 0.001). Similarly, the observed prevalence of having a SUD on two or more substances also exceeded chance expectations at both developmental stages. At each wave, the overall χ2 was highly significant (Wave 1: χ2 = 292.17, p<0.001; Wave 2: χ2 = 570.77, p < 0.001). Consequently, the elevated prevalence of the multiple SUD categories was balanced by the decline in the prevalence of the single SUD categories at each Wave. Further examination revealed that the observed prevalence of repeated-use and abuse of all three substances was increasingly greater than the expected prevalence as age increased.
