Figure 2 depicts the item characteristic curves, showing the relationship between sequential order of alcohol and cannabis use initiation in relation to overall substance use involvement. As illustrated, alcohol use was more likely than cannabis use across all levels of severity of substance use involvement. As expected, the probability of alcohol use is related to AUD risk (OR = 3.65, p < .001, 95% CI [2.41, 5.54]) and the probability of cannabis use is related to CUD risk (OR = 2.49, p < .001, 95% CI [1.75, 3.52]). More importantly, however, the TLI predicts the probability of both alcohol use (z = 2.94, p = .004) and cannabis use (z = 2.92, p = .004). Furthermore, probability of alcohol use mediates the relationship between TLI and AUD (z = 6.75, p < .001) and cannabis use mediates the relationship between TLI and CUD (z = 6.28, p < .001). Confirmation of common liability was further obtained by the finding that probability of alcohol use predicts CUD (OR = 2.75, p < .001, 95% CI [1.89, 4.01]) and probability of cannabis
