External validity and policy adaptation: From impact evaluation to policy design

* Whether a policy that worked in one context will also work in another depends on the interaction of the policy's mechanism with features of the context. This is its "external validity". * Existing methods are helpful for understanding a policy's external validity, but are limited to examining a small number of dimensions of context. * Mechanism mapping is a five-step procedure to help policymakers identify potential mechanism-context interactions, and come up with suitable adaptations.