The difference in weeks with substance use during follow-up was mediated by the impact of ACT on subsequent utilization of treatment services (point estimate = −0.48, 95% CI [−1.28, −0.08]). The significant differential impact of ACT on weeks of use, t(73) = −2.08, p < .05, became nonsignificant, t(73) = −1.55, p = .13, when accounting for the effect of treatment utilization (see Table 3; proportion of effect mediated = .25). When treatment utilization was examined as the outcome of interest and weeks of use tested as a mediator, it too was significant, but less strongly (point estimate = 1.75, 95% CI [0.15, 4.68]), and the differential impact of ACT on treatment utilization, t(73) = 2.22, p = .03, was closer to significance, t(73) = 1.72, p = .09, when accounting for weeks of use (see Table 3). Given the known importance of treatment utilization to outcomes (e.g., Ouimette, Moos, & Finney, 1998; Ritsher, Moos, & Finney, 2002), it seems more reasonable to suppose that treatment utilization may have helped decrease substance use; subsequently, posttreatment score mediators of subsequent treatment utilization were examined.
