Barkham, M., Lutz, W., Lambert, M., & Saxon, D. (2017). Therapist effects, effective therapists, and the law of variability. In L.G. Castonguay and C.E. Hill (Eds.) How and why are some therapists better than others? Understanding therapist effects. Washington, DC: American Psychological Association.
Psychotherapy research has often focused on the differences between treatment types (CBT, interpersonal psychotherapy, psychodynamic therapy), which has overshadowed research on what makes for an effective therapist. Psychotherapists represent the most costly important component of psychotherapy, and only recently has research begun to catch up to the importance of therapist effects. The term “therapist effects” refers to differences between therapists (i.e., variability) in their clients’ outcomes. In this chapter, Barkham and colleagues review some of the research on effective therapists. Best estimates of therapist effects suggest that differences between therapists account for about 8% of client outcomes – which is considered a medium effect and larger than the variance accounted for by the type of therapy that a client receives. Psychotherapy research often tries to control for therapist effects by training therapists to adhere to a manual, however adherence to a manual does not substantially reduce therapist effects, and adherence is not related to patient outcomes. The implication is that which therapist a client sees matters to the client’s mental health outcomes. The best research on the topic indicates that about 20% of therapists are substantially better than the average therapist, and 20% are substantially worse than the average. (The good news is that 60% of therapists [the average] are equally and positively effective). In that study of 119 therapists, the least effective therapists had about 40% of their clients recover, whereas the most effective therapists had about 76% of their clients recover. In other words, the better therapists were almost twice as effective as the worse therapists. In a re-examination of previous data, Barkham and colleagues looked at whether other variables, like client symptom severity, played a role in therapist effects. They found that differences among therapists was higher as client baseline severity increased. That is, the gap between better and worse therapists increased when client symptoms were more severe and complex. Good therapists were better equipped to handle more complex cases.
There are important differences between therapists in their effectiveness, and this makes a difference to clients. It is particularly important for clients with more severe symptoms to be matched with more effective therapists. Previous research indicates that the level of therapist interpersonal skills (alliance, empathy, warmth, emotional expression, verbal skills) can account for significant proportion of therapist effects, and so training therapists in these interpersonal skills will improve client outcomes. Also, therapists who receive continuous reliable feedback throughout therapy about their client’s symptom levels can also drastically reduce client drop-outs and the number of clients who get worse during treatment.