Evidential Pluralism was previously applied to produce an extension of evidence-based medicine that systematically scrutinises mechanistic studies alongside association studies. The resulting approach was called EBM+. In this chapter, we argue that an analogous move is warranted in the area of evidence-based policy. We show that Evidential Pluralism leads to a new account of evidence-based policy assessment, which we call EBP+. EBP+ provides the capability to assess mechanistic studies alongside the association studies that are the focus of present-day evidence-based policy assessment. This approach can lead to better-informed judgements of the effectiveness of social interventions, and thus to better social policy. We compare this new account to related approaches, such as realist evaluation.