The great potential for evidence to inform public policy Chapter 1 explained that evidence matters. In many ways, of course, this is self-evident. For any decision and for any course of action, we will want information that tells us if we are achieving our goals, or to inform our selection of possible strategies to achieve our goals. Evidence is, by definition, what tells us these things (even if there may be debate over which goals to pursue in the first place). And so, in public policymaking, evidence can be useful for any number of decisions – from those as mundane as changing the timing of traffic lights to those as profound as a decision to go to war. And yet the rhetoric about the need for governments to do ‘what works’ under the banner of ‘evidence-based policymaking’ (EBP) has seen particularly widespread growth in recent years (Davies, Nutley and Smith 2000b; Nutley, Walter and Davies 2007).