Differentiating Ambiguity: A Comment.

In Ghirardato, Macheroni and MArcinaccri (GMM) propose a method for distinguishing between percieved ambiguity and the decision-maker's reaction to it. They study a general class of preferences which includes CEU and a-MEU and axiomatise a subclass of a-MEU preferences. We show that for Hurwicz preferences the proposed measure of ambiguity depends on parameters which intuitively reflect ambiguity-attitude. Furthermore, any a-MEU preferences which satisfy the CEU axioms, satisfy GMM's axioms if and only if a = 0 or 1, that is, the capacity must be convex or concave.