| Summary: | Valid and reliable measures of Acceptance and Commitment Therapy's (ACT) purported mechanism(s) of change are paramount in establishing its incremental validity above other extant psychological therapies. However, the most widely used general measure of psychological flexibility and the six ACT processes of which it is composed of, the Acceptance and Action Questionnaire (AAQ-II), has been significantly criticised, principally in terms of being a conflated distress measure due to having items with poor face and content validity. Theoretically, there has also been debate about whether ACT's 'hexaflex' model, which conceptually underpins the AAQ-II and other ACT process measures, is comprised of fewer than six distinct processes as per current ACT theory. The overall aim of our study was therefore to: 1) develop a new general measure of ACT processes; 2) test the measure's validity and reliability; and 3) establish whether the measure's factor structure accords with Hayes et al.'s (2011) clustering of ACT's six processes into three dyadic processes. The measure was named the Comprehensive assessment of ACT processes ('CompACT'). Using a Delphi-consensus methodology, ACT experts (N= 10) rated the face and content validity of 106 items. Items were taken from existing ACT process measures and two mindfulness measures, as well as generated by ourselves and the ACT experts within our study. Items which failed to reach pre-defined ratings thresholds were removed and remaining items were re-rated until a pre-defined consensus threshold was reached. After three rounds, an initial 37-item version of the CompACT was produced. The CompACT was then disseminated to a non-clinical sample of participants (N = 377) mainly via social media. Participants also completed other measures to assess different aspects of the CompACT's validity. A three-factor structure was found to be the most stable for a 30-itemed version of the CompACT. An examination of the distribution and contents of item loadings also confirmed that the CompACT's three factors could be defined in accordance with Hayes et al's. (2011) three dyadic ACT processes. The CompACT performed in theoretically predicted ways against measures of other related and distinct constructs, and was mostly comparable to the AAQ-II in terms of its magnitude of associations with these other measures, with the exception of a distress measure. The CompACT's associations with these measures also supported ACT's central hypothesis: higher levels of psychological inflexibility are associated with higher levels of distress and lower levels of health and wellbeing. Overall, the CompACT shows initial promise as a comprehensive measure of ACT processes, and more generally as a measure of psychological flexibility. The psychometrically viability of Hayes et al.'s (2011) three dyadic ACT processes as a conceptual model for the CompACT further strengthens previous suggestions that ACT's clinical model may be composed of fewer than six distinct processes. The CompACT was adjudged by ACT experts to possess items with better face and content validity than the AAQ-II. However, in order to be considered as a viable alternative measure to the AAQ-II, further testing of the CompACT's psychometric properties is needed.
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