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.