Background: 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. Methods: 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. Results: The PS offered statistical control over observed pretreatment differences between patients in a non-randomized study. Conclusions: 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.