It should be noted that the median number of drug-use days was low and restricted, with an interquartile range between 1 and 3 days of self-reported use per month. Such a restricted range made it difficult to detect statistically significant or clinically meaningful differences in substance use trajectories. The overwhelming majority of adolescents in the study were referred from residential treatment or from juvenile justice, both of which involved surveillance (and limited opportunities to engage in drug use). These referral sources may have been responsible for the relatively low baseline rates of drug use,
and in the case of the juvenile justice referrals, continued surveillance may have been responsible for the low levels of drug use over time.