Data-Based Decisions under Imprecise Probability and Least Favorable Models

Data-based decision theory under imprecise probability has to deal with optimisation. problems where direct solutions are often computationally intractable. Using the Gamma-minimax optimality criterion, the computational effort may significantly be reduced in the presence of a least favorable model. In 1984, A. Buja derived a necessary and sufficient condition for the existence of a least favorable model in a special case. The present article proves that essentially the same result is valid in case of general coherent upper previsions. This is done mainly by topological arguments in combination with some of L. Le Cam's decision theoretic concepts. It is shown how least favorable models could be used to deal with situations where the distribution of the data as well as the prior is assumed to be imprecise.