Validation of the Marijuana Purchase Task Among Adolescent Marijuana Users

Marijuana is the most commonly used illicit substance, with daily use rising among 8th and 10th graders. Adolescents view marijuana use as carrying minimal risk despite a host of associated psychosocial consequences. Within the behavioral economic framework, the Marijuana Purchase Task (MPT) has been used to understand demand (i.e., relative value) for marijuana. The MPT is a hypothetical purchase task that produces five demand indices reflective of the relative reinforcing value of marijuana and include intensity (i.e., amount consumed at zero cost), Omax (i.e., maximum expenditure), Pmax (i.e., price at maximum expenditure), breakpoint (i.e., cost at which consumption is suppressed to zero), and elasticity (i.e., rate at which consumption decreases as price increases). To date, the MPT has only been validated with adult samples; thus, the current study aimed to validate the MPT with a late adolescent sample who presumably have less experience in purchasing and using marijuana relative to adult users. Convergent validity was established via correlations between demand indices and marijuana outcomes (i.e., marijuana use, consequences, craving, and recent marijuana expenditures). Divergent validity was established via t-tests to examine group differences between hazardous and non-hazardous users as differentiated by the Cannabis Use Disorders Identification Test-Revised (CUDIT-R; scores of 8 or greater indicate hazardous use). Participants were 115 adolescents (Mage = 16.94, SDage = 0.88; 52% female; 64% high school student) between the ages of 15-18 who reported lifetime marijuana use and current marijuana demand. As expected, results showed that as price increased, hypothetical marijuana use decreased. Convergent validity was established via significant associations between demand indices and marijuana outcomes. Omax, breakpoint, and elasticity were significantly correlated with marijuana use outcomes in predicted directions such that greater demand was associated with more use, consequences, craving, and recent expenditures (ps <.05). Intensity was positively correlated with craving and expenditures (ps < .05). Pmax was not significantly correlated with any marijuana use outcome. Divergent validity was also established; compared to non-hazardous users (n = 39), hazardous users (n = 76) exhibited significantly higher Omax (t = 3.11, p <.01), Pmax (t = 2.08, p <.05), breakpoint (t = 3.71, p < .001), and elasticity (t = 3.11, p <.01). There was no difference in intensity across user types. Findings from the current study are unique in several ways. First, in contrast with previous literature, intensity was less consistently associated with marijuana outcomes. However, indices related to price sensitivity are important metrics in this age group, as evidenced by significant associations between Omax, breakpoint, and elasticity and marijuana outcomes. These findings are further evidenced by the ability of Omax, Pmax, breakpoint, and elasticity to differentiate non-hazardous versus hazardous users. Together, these findings suggest that the MPT is a valid measure for assessing the reinforcing value of marijuana among adolescents. Future research should replicate these findings, as well as examine the factor structure of the MPT among adolescents.