A fully nested MMRM analysis showed no effect for treatment condition (p = .83), but there was a significant effect for time (p < .001) and the interaction of condition and time, F(1, 99.60) = 3.59, p = .03, effect size = .38 (a small effect). Contrary to expectation, TAU participants showed a medium and significant improvement from pre- to posttreatment, Mdiff = −11.47, SE = 2.99, t(119.10) = −0.84, p = .00, 95% CI [−17.39, −5.55], effect size = .51, but this decreased to a small and nonsignificant change from pretreatment to follow-up, Mdiff = −6.25, SE = 4.25, t(86.67) = −1.47, p = .15, 95% CI [−14.71, 2.19], effect size = .22. Conversely, participants in the ACT condition showed a small and significant improvement from pre- to posttreatment, Mdiff = −5.78, SE = 2.85, t(117.31) = −2.03, p = .045, 95% CI [−11.43, −0.13], effect size = .26, which increased to a medium and significant improvement through the follow-up period, Mdiff = −18.55, SE = 4.38, t(86.89) = −4.23, p = .000, 95% CI [−27.26, −9.83], effect size = .66. The effect is shown visually in Figure 3, along with the other psychosocial outcomes.
