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Background: Variations in the delivery of content and process can alter the effectiveness of complex interventions. This study examined the fidelity of a weight loss intervention (Camden Weight Loss) from recorded consultations by assessing advisors' delivery of content, use of motivational interviewing approach and therapeutic alliance.
Methods: A process evaluation was conducted of advisor-participant consultations in a 12-month randomised controlled trial of an intervention for adult volunteers with a body mass index categorised as overweight or obese. A convenience sample of 22 consultations (12% of 191 participants) recorded at the intervention mid-point were available for analysis. Consultations were independently rated by two observers independent of intervention or study delivery, using: a fidelity scale, the Motivational Interviewing Treatment Integrity Scale and the Primary Care Therapy Process Rating Scale. Raters were blind to participants' responses to the intervention and weight outcomes. Half the participants ( = 11) achieved significant weight loss (≥ 5% of baseline weight).
Results: A mean of 41% of prescribed content was delivered, with a range covered per session of 8-98%, falling below the 100% content expected per session. Tasks included most frequently were: taking weight and waist measurements (98%), scheduling next appointment (86%), review of general progress (85%) and reviewing weight change (84%). Individual items most frequently addressed were 'giving encouragement' and 'showing appreciation of participant's efforts' (95 and 88% respectively). Consultation length (mean 19 min, range 9-30) was shorter than the 30-min allocation. Quantity of content correlated with consultation length ( < 0.01). Advisors' use of motivational interviewing was rated at 'beginner proficiency' for Global Clinician Rating, Reflection to Question Ratio and Percent Open Questions. Therapeutic alliance scores were moderate. Affective aspects were rated highly (e.g. supportive encouragement, involvement and warmth).
Conclusions: Intervention fidelity varied in both content and process, emphasising the importance of ongoing fidelity checks in a complex intervention. Advisors focused on certain practical aspects of the intervention and providing an encouraging interpersonal climate. This concurs with other research findings, which have revealed the value participants in a weight loss intervention place on an empathic advisor-participant relationship.
Clinical Trials Registration: Registered with Clinicaltrials.gov, number NCT00891943, on 1 May 2009.
