title: Acceptance and commitment therapy: a meta-analytic review.
abstract: BACKGROUND
  There are now a substantial number of controlled trials investigating the efficacy of acceptance and commitment therapy (ACT). This meta-analysis combined multiple well-controlled studies to help clarify the overall impact of ACT relative to waiting lists, psychological placebos, treatment as usual, and established therapies.
  
  
  METHODS
  A comprehensive literature search produced 18 randomized controlled trials (n = 917) that were included in the final analyses. Effect size was computed with Hedges's g which can be interpreted with Cohen's convention of small (0.2), medium (0.5), and large (0.8) effects.
  
  
  RESULTS
  There was a clear overall advantage of ACT compared to control conditions (effect size = 0.42). The average ACT-treated participant was more improved than 66% of the participants in the control conditions. Analyzed separately ACT was superior to waiting lists and psychological placebos (effect size = 0.68) and treatment as usual (effect size = 0.42). However, ACT was not significantly more effective than established treatments (effect size = 0.18, p = 0.13). Also, ACT was not superior to control conditions for the distress problems (anxiety/depression: effect size = 0.03, p = 0.84).
  
  
  CONCLUSIONS
  The results reveal that ACT is more effective than control conditions for several problem domains, but there is no evidence yet that ACT is more effective than established treatments.
authors.affiliation: Faculty of Social and Behavioral Sciences, Clinical Psychology, University of Amsterdam, Amsterdam, The Netherlands. markpow@mail.med.upenn.edu
authors.forename: Mark B
authors.initials: MB
authors.lastname: Powers
authors.forename: Maarten B
authors.initials: MB
authors.lastname: Zum Vorde Sive Vording
authors.forename: Paul M G
authors.initials: PM
authors.lastname: Emmelkamp
mesh_terms: D015928:Cognitive Behavioral Therapy; D006801:Humans; D011602:Psychophysiologic Disorders / Q000628:therapy*