Communicating uncertainty in policy analysis.

The term "policy analysis" describes scientific evaluations of the impacts of past public policies and predictions of the outcomes of potential future policies. A prevalent practice has been to report policy analysis with incredible certitude. That is, exact predictions of policy outcomes are routine, while expressions of uncertainty are rare. However, predictions and estimates often are fragile, resting on unsupported assumptions and limited data. Therefore, the expressed certitude is not credible. This paper summarizes my work documenting incredible certitude and calling for transparent communication of uncertainty. I present a typology of practices that contribute to incredible certitude, give illustrative examples, and offer suggestions on how to communicate uncertainty.