Longitudinal measurement invariance of the Working Alliance Inventory - Short form across coaching sessions

Background: Throughout the psychotherapeutic and coaching literature, the client-therapist or coach-coachee working alliance has been highlighted as key force driving positive outcome. The Working Alliance Inventory Short form (WAI-S) for coaching charts the quality of working alliance throughout coaching sessions and is broadly applied in coaching research. Due to a shortfall in research on psychometric properties of the WAI-S, the purpose of this study was to examine (a) if the theorized three-factor structure of the 12-item WAI-S forms a solid representation of the dimensions of working alliance in coaching, and (b) longitudinal measurement invariance (LMI) of the WAI-S. Method: Data were collected in a two-wave study design comprising a main study sample of N = 690 Dutch coachees that completed the questionnaire at the first measurement, of which N = 490 also completed the questionnaire at the second measurement. Post hoc sensitivity analysis was performed based on the original sample, lacking additional information on covariates, and included both completers and dropouts, comprising N = 1986 respondents at T1, and N = 1020 respondents at T2. Results: Confirmatory factor analyses evidenced best fit of the three-factor model in comparison to one-, and two-factor models at both time points. Despite the fact that multigroup confirmatory factor analysis detected non-invariant intercepts, our findings overall supported measurement invariance across coaching sessions. Conclusions: As decisions in both clinical and scientific practices generally rely on outcome assessment of interpersonal change in scores on the same measure over time, we believe our findings to be of contributing value to the consolidation of interpretation and accuracy of scorings on the WAI-S in coaching.