The original recommendations on evidence-based treatment from the American Psychological Association (Chambless, 1995) began to be criticized as their definition of ESTs, i.e., Empirically Supported Treatments, was too focused on efficacy studies, namely RCTs. Furthermore, clinical expertise and patient choice (Sackett et al., 1996) had been ignored in the recommendations. There was also a devaluation of evidence other than that demonstrated through RCTs. The internationally respected psychotherapy researcher Rolf Sandell (2006) points out that there is now “extensive research showing the relatively greater importance for treatment outcomes of other factors (patient, therapist, treatment alliance, and other so-called common factors) than the treatment method itself” (26).

Falk Leichsenring, another renowned therapy researcher, states: “After more than half a century of research, thousands of RCTs and millions of invested funds, the effect sizes of psychotherapies and pharmacotherapies for mental disorders are limited, suggesting a ceiling effect for treatment research as presently conducted” (Leichsenring et al., 2022)(133). Therefore, there has been an interest in the factors Sandell mentioned, i.e., that outcomes are not so much influenced by method (PDT, CBT, DBT, ACT, etc.) as by other variables such as patient, therapist, and their working alliance.