Weight loss maintenance in adults: the WILMA trial process evaluation

Abstract

Weight loss maintenance interventions have had limited effectiveness. We assessed an intervention based on Motivational Interviewing (MI) in a 3-arm RCT. This paper outlines the trial process evaluation framework, presents the results of the fidelity assessment and discusses key challenges in measuring fidelity. We developed a mixed methods process evaluation framework and a logic model which outlined the components of the intervention and the processes by which the intervention brings about change through theorised mediators. The intervention was successfully delivered by trained counsellors. There were some issues with telephone delivery and use of the website for self-monitoring. The data gave some support to the theorised intervention model. The MI was delivered with good fidelity, although assessing fidelity raised a number of issues including obtaining recordings of sessions, over-manualising, identifying a gold standard, cost and the degree of flexibility in delivery that is acceptable. These methods are crucial for understanding how interventions are implemented and how they produce change and there are many issues to be considered at the design stage, in particular in relation to assessing fidelity.