Fig. 4. Conceptual models of single-step decision making. In the various parts of this figure, X represents the system state, X₀ an observed aspect of the system state, D a decision, X₁ the system state following the decision, u a value or utility, * the output from a single model, ** the output from an alternative model, t time, and M a model describing the relationship between X, D, and u. This figure is discussed in some detail in the Decision making section.