Using the matrixed multiple case study methodology to understand site differences in the outcomes of a Hybrid Type 1 trial of a peer-led healthy lifestyle intervention for people with serious mental illness.

A healthy lifestyle intervention delivered by people with lived experience of serious mental illness (SMI) led to improved health outcomes among participants living with SMI in three supportive housing agencies, but these outcomes differed by study sites. This study aimed to identify which factors influenced these site differences in study outcomes by examining if the intervention was delivered as intended, intervention recipients' satisfaction with the intervention and the pre-existing health care services delivered at each study site, and what changes occurred in health care services offered at each of the supportive housing agencies throughout the study. The intervention was found to have high levels of satisfaction across all three agencies but the agency which had the best outcomes also had the highest levels of intervention delivery adherence and the lowest levels of intervention recipients' satisfaction with existing health care services. Additionally, during the study period, the same agency described minimal changes in their existing health care services while the other two agencies were found to have made more significant changes integrating health care services to their day-to-day operations. As a result, findings from this study emphasize the importance of understanding the context in which interventions are delivered in routine practice settings to ensure their overall success.