In addition to changes from pre to post, we also examined controlled effect sizes. These effect sizes were smaller but still significant (Hedges’ g = 0.50 and 0.81 for reducing symptoms of depression and anxiety in active treatment controlled studies, and Hedges’ g = 0.41 and 0.32 in waitlist and TAU controlled studies). However, the fail-safe N analysis suggested that, except for measures of anxiety symptom severity in active controlled studies, the results of the controlled effect size analyses were unreliable due to the small number of studies. Similarly, although significant, the ITT effect sizes (Hedges’ g = 1.06 and 0.55 for reducing symptoms of depression and anxiety, respectively) should only be considered preliminary. In contrast, the pre-post effect sizes were robust. A meta-analysis of the effects of psychological placebo conditions in anxiety disorder trials (Smits & Hofmann, 2009) yielded a pre- to post-treatment effect size (Hedges’ g) of 0.45 (95% CI: 0.35-0.46), suggesting that the effect sizes associated with MBT are significantly greater than the placebo effect size.
