Treatment sensitivity: Its importance in the measurement of psychological flexibility

Background: Psychological flexibility (PF) is a central construct in Acceptance and Commitment Therapy (ACT). Many studies have operationalized PF using the self-report Acceptance and Actions Questionnaire (AAQ-II). Information on the treatment sensitivity of self-report assessments of PF is lacking, however. We investigated differences in the treatment sensitivity of the AAQ-II compared to other measures of PF across various samples. Methods: Using three different clinical samples (N = 164), we compared the pretreatment-posttreatment change scores of the AAQ-II to those of three alternative self-report questionnaires measuring PF in a within-subject design. Sensitivity to change was assessed with effect sizes and Reliable Change Index (RC). Results: Without exception, effect sizes and rates of clinically significant change were larger in all three alternative questionnaires and across three populations compared to the standard formulation of the AAQ-II. Conclusions: The results of the present study show greater treatment sensitivity of three alternative questionnaires measuring PF compared to the AAQ-II. The results suggest that treatment effects concerning PF may have been underestimated depending on the wording and measure used. Implications for research on PF and ACT processes and outcomes are discussed.