Welldefined Decision Scenarios

Influence diagrams serve as a powerful tool for modelling symmetric decision problems. When solving an influence diagram we de­ termine a set of strategies for the decisions involved. A strategy for a decision variable is in principle a function over its past. How­ ever, some of the past may be irrelevant for the decision, and for computational reasons it is important not to deal with redundant vari­ ables in the strategies. We show that current methods (e.g. the Decision Bayes-ball algo­ rithm [Shachter, 1998]) do not determine the relevant past, and we present a complete al­