Objective: To introduce readers to instrumental variable analyses for causal inferences using as an example a test of the hypothesis that the quality of the therapeutic alliance has a causal role in relation to the outcome of psychotherapy. Method: We used data from a recent non-inferiority trial of cognitive and dynamic therapies for major depressive disorder in a community mental health setting. The data (N = 161) were analyzed using standard approaches as well as a multilevel 2-stage instrumental variables approach that allows for causal interpretations by removing the influence of unmeasured confounds. Results: Instrumental variables were created at the patient and therapist level using baseline patient and therapist variables. These baseline variables predicted the alliance but were otherwise unrelated to treatment outcome other than through their effects on the alliance. Standard multilevel mixed effects analyses revealed statistically significant associations of the alliance with outcome at the therapist level of analysis. The therapist level effect remained statistically significant when using the instrumental variables approach. Conclusion: Our results support the hypothesis that, at least at the therapist level, the alliance plays a causal role in producing better outcomes. Instrumental variable analyses can be a useful tool to supplement standard analyses.
Keywords: alliance; causal inference; depression; instrumental variables; outcome.