Discussion of a technique to reduce the long simulations that are otherwise necessary before rare, interesting events occur and can be studied. The technique is based on "virtual" measures, characterized as qualities that depend on but do not alter the system image at different points in simulated time. Using a virtual measure in estimating a system characteristic may yield an estimator that has a variance an order of magnitude smaller than would result from direct measurements from the same simulation run. It often takes only nominal extra effort to calculate the virtual measure and its associated estimator, so the total computer time needed to achieve statistical reliability may be greatly reduced. The technique has been found useful in studying how to deploy New York City firefighting units so as to minimize loss of life in fires. 24 pp. Ref.
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