Abstract
OBJECTIVES: Understanding the fidelity of delivery of complex health behaviour interventions is crucial in determining their effectiveness and identifying aspects needing refinement. PROGROUP is a group-based intervention for people with severe obesity. It aims to promote a shared social identity to support behaviour change. Data from a feasibility randomized controlled trial (fRCT) were used to assess fidelity of intervention delivery and the impact on patient experiences, to optimize the intervention for a main trial. METHODS: Data from 18 patient and five facilitator interviews, audio and video data of group sessions, two fidelity checklists, support calls and a group processes questionnaire were used to assess fidelity of delivery to intervention principles, patients' experience of the intervention and areas for optimization. RESULTS: The number of activities delivered and facilitator confidence and rapport with the group affected fidelity to intervention principles. The facilitators' delivery style, group composition and attendance affected the groups' sense of social identity. Accordingly, the intervention content was revised to ensure better balance between educational material and group activities, to increase facilitator confidence and enable flexible delivery. CONCLUSIONS: The success of group-based interventions relies on the facilitator addressing the group's needs and creating conditions for a shared social identity to develop. Assessment of fidelity to the manual content and core function of PROGROUP enabled identification of components needing refinement, incorporating both facilitator and patient perspectives. The assessment and optimization process offer a blueprint for evaluating other group-based interventions.