External validity and policy adaptation : a five - step guide to mechanism mapping

With the recent boom in impact evaluations around the world, policymakers in many sectors now have at their disposal an overwhelming amount of evidence about “what works” or at least what worked in a particular context. Yet as impact evaluations have multiplied, it has become apparent that “the same” policy can have very different effects in different populations (Vivalt 2016). Similarly, policies shown to be effective in small trials have not always been as effective when implemented at scale, even in the same country (Bold et al 2016).