This paper engages with the continuing emphasis given to evidence-based policy and "what works" approaches in educational research, highlighting some of the continuing epistemological challenges from a post-positivist perspective. To illustrate these, it uses the case of bursaries awarded by universities to improve outcomes for students from disadvantaged backgrounds as an example of an education intervention designed to address structural inequality. The paper then discusses critical reflections arising from a project commissioned by the Office for Fair Access in England, which aimed to enable universities to evaluate the impact of the bursaries that they award. These reflections provide a lens to explore the limitations of experimental and quasi-experimental designs in complex social fields. The paper concludes that we lack a strong understanding of the relationship between financial and educational disadvantage prior to and during higher education, and this undermines efforts to "prove" that certain interventions will "level the playing field".