In search of synergy: strategies for combining interventions at multiple levels.

The social ecological perspective provides a compelling justification for multilevel intervention. Yet, it offers little guidance for selecting interventions that work together in complementary or synergistic ways. Using a causal modeling framework, we describe five strategies for increasing potential complementarity or synergy among interventions that operate at different levels of influence: accumulation, amplification, facilitation, cascade, and convergence. We illustrate these strategies with examples of multilevel interventions to improve the quality of cancer treatment.