Ambiguous beliefs`` are beliefs which are inconsistent with a unique, additive prior. The problem of their update in face of new information has been dealt with in the theoretical literature, and received several contradictory answers. In particular, the ``maximum likelihood update`` and the ``full Bayesian update`` have been axiomatized. This experimental study attempts to test the descriptive validity of these two theories by using the Ellsberg experiment framework.
Keywords. Decision Making, Uncertainty, Capacities, Updating, Conditioning rules.
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