The use of propensity score methods in psychotherapy research. A practical application

Randomized controlled trials are considered the best scientific proof of effectiveness. There is increasing concern, though, about their feasibility in psychotherapy research. We discuss a quasi-experimental study design for situations in which a randomized controlled trial is not feasible. Here, as an alternative strategy, the propensity score (PS) method is used to correct for selection bias.

We used data from a Dutch research project, SCEPTRE (Study on Cost-Effectiveness of Personality Disorder Treatment). The sample consisted of 749 psychotherapy patients with personality pathology. We tested whether the PS method was useful and applicable. We examined differences between 2 treatment groups (short vs. long treatment duration) in pretreatment characteristics before and after PS correction. This revealed the impact of the PS on outcome differences.

The PS offered statistical control over observed pretreatment differences between patients in a non-randomized study.

When a randomized controlled trial is not possible, this quasi-experimental design using the PS could be a feasible alternative. Its advantages and limitations are discussed. Implemented carefully, this method is promising for future effectiveness research.