ope ration, an organiz ation, or a systezn. This often leads to a zneasure of znerit or a value parazneter by which operational results can be coznpared; for exaznple, the fraction of approaching aircraft shot down by an air-defense systezn, or annual sales of products by an industrial con­ cern, or gross national product of a country. Predictions of such operational results, and quanti­ tative coznparisons between different systezns, require znatheznatical znodels of each systezn and its operation. In sozne cases--such as the accuracy of anti-aircraft fire, or the cost of producing a znachined product, or transportation costs of delivery --the znodel can be znatheznatically precise or deterzninistic. More generally, operational results are probabalistic and require a stochastic znodel.