Examining Item Content Across Nine Psychological (In)flexibility Scales: What Do They Measure?

Psychological inflexibility and flexibility are central to the model of acceptance and commitment therapy (ACT). As such, it is critical to assess these constructs accurately and reliably to provide robust tests of ACT theory. One measurement aspect to consider is the content covered by scales purported to assess the same construct. The present study provides a description of content included in nine psychological flexibility and inflexibility scales and evaluates the degree of content overlap across scales using the Jaccard Index. Overall, we found that the scales overwhelmingly focused on internal experiences (78.5% of items), with the most common type being emotions/feelings (41.1%) and thoughts (28.2%). In addition, the Jaccard Index indicated very weak overlap with respect to item content, suggesting that the constructs of psychological flexibility and inflexibility are measured inconsistently, compromising our ability to pool data from different studies. The Comprehensive Assessment of ACT Processes (CompACT) and Multidimensional Psychological Flexibility Inventory (MPFI) had the highest mean overlap scores. Despite broad consensus, the lack of overlap when considering precise categories and item content underscores the need to clarify definitions of psychological flexibility and inflexibility for measurement and practice greater transparency when reporting on study measures.