Threat alert: The effect of outliers on the alliance-outcome correlation

Meta-analyses have established the alliance as the most robust predictor of outcome in psychotherapy. A growing number of studies have evaluated potential threats to the conclusion that alliance is a causal factor in psychotherapy. One potential threat that has not been systematically examined is the possibility that the alliance-outcome association is driven by low alliance outliers. We examined the influence of removing low alliance outliers on the alliance-outcome association using data drawn from two large-scale, naturalistic psychotherapy data sets (Ns = 1,052; 11,029). These data sets differed in setting (university counseling center, community mental health center), country (United States and Canada), alliance measure (four-item Working Alliance Inventory Short Form Revised, 10-item Session Rating Scale), and outcome measure (Counseling Center Assessment of Psychological Symptoms-34, Outcome Questionnaire-45). We examined the impact of treating outliers in five different ways: retaining them, removing values three or two standard deviations from the mean, and winsorizing values three or two standard deviations from the mean. We also examined the effect of outliers after disaggregating alliance ratings into within-therapist and between-therapist components. The alliance-outcome correlation and the proportion of variance in posttest outcomes explained by alliance when controlling for pretest outcomes were similar regardless of how low alliance outliers were treated (change in r ≤ .04, change in R² ≤ 1%). Results from the disaggregation were similar. Thus, it appears that the alliance-outcome association is not an artifact of the influence of low alliance outliers. (PsycInfo Database Record (c) 2023 APA, all rights reserved).