Implementing a complex psychosocial intervention for unstably housed Veterans: A realist-informed evaluation case study.

Implementation is inherently dynamic and influenced by interdependent factors operating at the individual, organizational, and system levels. This is especially true for complex interventions addressing co-occurring mental health and substance use disorders because such interventions involve multiple treatment modalities delivered simultaneously, in busy practice settings, with challenging populations. This paper pairs consolidated framework for implementation research (CFIR) constructs with a realist evaluation approached to generate configurations important to the adoption, implementation, and adaptation stages of a highly complex intervention addressing the behavioural health and housing needs of a vulnerable population. Each configuration describes how contextual factors trigger mechanisms to generate implementation outcomes and answers "what works for whom, in what circumstances and in what respects, and how?" These findings further our understanding of possible mechanisms of change and push us to be more precise about identifying causal relationships among constructs that contribute to the success of implementing complex interventions. This work also moves us to think theoretically and methodologically in a more dynamic fashion, thereby leading to more responsive implementation practice.