A decision maker constructs a convex set of nonnegative martingales to use as likelihood ratios that represent alternatives that are statistically close to a decision maker's baseline model. The set is twisted to include some specific models of interest. Max-min expected utility over that set...

A decision maker suspects that parameters of a set of structured parametric probability models vary over time in unknown ways that he does not describe probabilistically. He expresses a fear that all of these parametric models are misspeci ed by also wanting to consider alternative unstructured...

Geophysicists examine and document the repercussions for the earth's climate induced by alternative emission scenarios and model specifications. Using simplified approximations, they produce tractable characterizations of the associated uncertainty. Meanwhile, economists write highly stylized...

We live in a world surrounded by uncertainty. In this essay, I show that featuring this phenomenon more in economic analyses adds to our understanding of how financial markets work and how best to design prudent economic policy. This essay explores methods that allow for a broader...