Abstract
Advocates of evidence-based policy making (EBPM) are typically concerned with the impact of particular interventions. This implicit ontology of the policy world, as disaggregated into a variety of independent interventions, has been challenged by Pawson (2006), in terms of the contingencies that activate, inhibit or reshape the impact of any intervention. This critique does not, however, go far enough. The identity, potentialities and impact of an intervention are contingent on the synergies that it develops with other interventions. This ontology of evolutionary or transformative realism provides the vantage point for reconsidering the theory and practice of EBPM and systematic review.