This article presents a realist-informed programme-level theory of change developed for a multi-site evaluation of a focused deterrence intervention aimed at reducing serious violence in five UK cities. Focused deterrence, a complex, cross-agency approach, requires theoretical tools that can account for local variation, emergent mechanisms and shifting implementation contexts. Using a five-stage process involving document review, fieldwork, workshops and qualitative interviews, we developed and refined the realist-informed programme-level theory of change to reflect variation in delivery, updated assumptions and context–mechanism–outcome configurations. Our findings reveal divergent delivery models, re-interpretations of core intervention resources and associated mechanisms, non-linear behavioural trajectories and participants’ strategic responses to perceived risks and opportunities. The final model offers a transferable framework for understanding and evaluating how complex interventions unfold across systems. We conclude by outlining lessons for evaluators seeking to develop theory-informed, complexity-aware theories of changes in real-world settings. These are particularly relevant in contexts involving cross-sector coordination, multiple delivery systems and flexible but systematic evaluation designs.
McFarlane et al. (2026) studied this question.