"Governments, aid donors and the development community at large are increasingly asking for hard evidence on the impacts of public programs claiming to reduce poverty. Do we know if such interventions really work? How much impact do they have?" Mobilised by these questions, Martin Ravallion uses this paper to critically examine what he calls "the archetypal evaluation problem": impact evaluation (or "counterfactual analysis") often does not strongly attribute observed outcomes to the specific programme being evaluated.