The dodo bird might be extinct in the real world but in the world of psychotherapy research it refuses to die. However, a group of German researchers recently put forward an article were they had randomized patients to either a PDT or CBT condition and measured the relative proficiency of the two orientations, and they found that their results delivered a convincing blow to the dodo bird verdict.

Introduction

The dodo bird verdict is the long held belief by some researchers and clinicians that all psychological interventions produce the same outcome. The proponents of this theory often attribute the efficacy of a therapy to “common factors”, such as the alliance between therapist and client. This is in opposition to crediting the success of a therapy to the specific techniques used by the therapist, e.g. exposure or cognitive restructuring.

It’s no secret that the main rivalry has been between cognitive behavior therapy (CBT) and psychodynamic therapy (PDT) practitioners. However, one of the issues with testing the dodo bird verdict has been the lack of quality studies on the efficacy of PDT. More importantly very few direct comparisons have been made within randomized controlled trails. Therefore it’s always exciting when a research group puts forward precisely that.

The study

Watzke et al. (2012) recruited 189 patients and randomized them to either CBT or PDT, they used a 3:2 ratio because the facilities had less capacity to give PDT. The study took place in a natural setting at an inpatient unit in Germany, and because of the naturalistic setting very broad inclusion criteria were used. The treatments were administered as brief group therapies, with 3-4 sessions per week and an average treatment length of 6 weeks. Both treatment groups additionally received one individual session per week. No treatment manuals were used and the therapists received no special training for this study.

The primary outcome used in the study was the General Severity Index of the Symptom Check list-14 (SCL-14). The secondary outcomes was mental component summary of the SF-8, and the Inventory of Interpersonal Problems (IIP-C). The outcomes were assed at intake and at 6 month follow up.

You can see their results in Figure 1, were I’ve made a plot of the results, and calculated Cohen’s d with 95 % CIs.

Figure 1. A Comparison of Cognitive behavioral therapy (CBT) vs Psychodynamic therapy (PDT) in randomized controlled trail (RCT)

Conclusion

In this direct comparison between CBT and PDT, CBT clearly performed better, and quite convincingly so. Clearly the dodo bird did not fair well in this study, but more research like this is needed before the dodo bird finally can be put to rest. If indeed CBT is more effective than PDT, then this is incredible valuable research for all the patients out there. Hopefully we’ll see more randomized controlled trails that compare two bona fide psychotherapies in the future.

Quality of the evidence

This study is a randomized controlled trail and as such the evidence has got the potential to be of high quality. Some aspects of the study are a bit unclear though, for instance the authors never describe how the allocation sequence was concealed. And I couldn’t see any information on why they only hade two time points (baseline and 6 months), more time points would’ve provided more information. Additionally, I think they should’ve analyzed their data as a multilevel model, especially if they used many different therapists and different hospital units. Also, they did not state how many therapists that were used in the study, and consequently they did not test for any therapist interaction effect. However, the authors explicitly state that the treatments were not recorded and hence no assessment of adherence or competence was made. Though, the “results of a prior study including independent expert raters (video ratings) describe the main interventions of both treatments (Watzke et al, 2004, 2008) and suggest that there was sufficient treatment differentiation between the two treatments in the unit”. So it’s possible that the outcome is due to the CBT therapists being more competent, and not due to CBT being more effective as a specific intervention. However, to me this seems redundant as from my perspective it’s a sign of competence to choose CBT before PDT. This statement might seem unnecessary polemic, but CBT is the treatment of choice for many disorders today with a vast amount of research supporting its efficacy, so perhaps one must be a bit scientific naïve to administer PDT for diagnoses were research support is lacking.

The researchers used unequal groups, with a 2:3 ratio (CBT having more patients). It’s hard to estimate if this had any effect on the outcome, statistically this might affect the significance test if the assumptions of homogeneous variance is violated. However, by looking at the standard deviations reported in the study this doesn’t seem to be a problem. Moreover, the authors performed analysis to test if any cofounders might have been unequally distributed between the treatments, and found no evidence for this. Also, attrition didn’t seem to be a problem, the authors performed sensitivity analyses and intention-to-treat analyses, which did not reveal any cause for concern.

I’m not sold on the outcomes they assessed. I’m thinking that they could’ve used more outcome measures, for instance they could’ve assessed depression and anxiety separately. However, there’s evidence that the SCL-scales are quite good at detecting general symptom severity.

Overall, I find the results of this study interesting and I don’t think any of the study’s shortcomings invalidates its findings. But as always more studies are needed before any robust conclusion can be made.

Watzke B, Rüddel H, Jürgensen R, Koch U, Kriston L, Grothgar B, & Schulz H (2012). Longer term outcome of cognitive-behavioural and psychodynamic psychotherapy in routine mental health care: Randomised controlled trial. Behaviour research and therapy, 50 (9), 580-587 PMID: 22750189