Background: This study explores the challenges clinical teachers face when first using a prospective entrustment-supervision (ES) scale in a curriculum based on Entrustable Professional Activities (EPAs). A prospective ES scale has the purpose to estimate at which level of supervision a student will be ready to perform an activity in subsequent encounters. Methods: We studied the transition to prospective assessment of medical students in clerkships via semi-structured interviews with twelve purposefully sampled clinical teachers, shortly after the introduction of a new undergraduate EPA-based curriculum and EPA-based assessment employing a prospective ES scale. Results: While some clinical teachers showed a correct interpretation, rating strategies also appeared to be affected by the target supervision level for completion of the clerkship. Instructions to estimate readiness for a supervision level in the future were not always understood. Further, teachers' interpretation of the scale anchors relied heavily on the phrasing. Discussion: Prospective assessment asks clinical teachers to make an extra inference step in their judgement process from reporting observed performance to estimating future level of supervision. This requires a change in mindset when coming from a retrospective, performance-oriented assessment method, i.e., reporting what was observed. Our findings suggest optimizing the ES-scale wordings and improving faculty development
