The mathematical description of (usually) a real-world problem. In operations research/management science, mathematical models take on varied forms (linear programming, queueing, Markov systems, etc.), many of which can be applied across application areas. The basic OR/MS mathematical model can be described as the decision problem of finding the maximum (minimum) of a measure of effectiveness (objective function) E = F(X, Y), where X represents the set of possible solutions (alternative decisions) and Y the given conditions of the problem. Although a rather simple model in its concept, especially since it optimizes a single objective, this mathematical, decision model underlies most of the problems that have been successfully formulated and solved by OR/MS methodologies. Decision problem; Deterministic model; Stochastic model.