Fidelity is not easy! Challenges and guidelines for assessing fidelity in complex interventions

BACKGROUND: Fidelity in complex behavioural interventions is underexplored and few comprehensive or detailed fidelity studies report on specific procedures for monitoring fidelity. Using Bellg’s popular Treatment Fidelity model, this paper aims to increase understanding of how to practically and comprehensively assess fidelity in complex, group-level, interventions. APPROACH AND LESSONS LEARNED: Drawing on our experience using a mixed methods approach to assess fidelity in the INFORM study (Improving Nursing home care through Feedback On perfoRMance data—INFORM), we report on challenges and adaptations experienced with our fidelity assessment approach and lessons learned. Six fidelity assessment challenges were identified: (1) the need to develop succinct tools to measure fidelity given tools tend to be intervention specific, (2) determining which components of fidelity (delivery, receipt, enactment) to emphasize, (3) unit of analysis considerations in group-level interventions, (4) missing data problems, (5) how to respond to and treat fidelity ‘failures’ and ‘deviations’ and lack of an overall fidelity assessment scheme, and (6) ensuring fidelity assessment doesn’t threaten internal validity. RECOMMENDATIONS AND CONCLUSIONS: Six guidelines, primarily applicable to group-level studies of complex interventions, are described to help address conceptual, methodological, and practical challenges with fidelity assessment in pragmatic trials. The current study offers guidance to researchers regarding key practical, methodological, and conceptual challenges associated with assessing fidelity in pragmatic trials. Greater attention to fidelity assessment and publication of fidelity results through detailed studies such as this one is critical for improving the quality of fidelity studies and, ultimately, the utility of published trials. TRIAL REGISTRATION: ClinicalTrials.gov NCT02695836. Registered on February 24, 2016 SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13063-021-05322-5.