The important concept which a computer enables an engineer or a policy maker to perform is the computation of an outcome from a simulation model. This allows an engineer or policy maker to analyze different cases without having to build a prototype or to perform experiments. In the design of a car or aircraft wing, the air resistance can be determined by a computer simulation program without having to perform a wind tunnel experiment. Similarly, an economist can forecast the consequences of alternative monetary policies. The value of simulation programs depends on how well the theory upon which the model is built explains the underlying phenomenon. Simulation models based on theories from natural science are generally much more accurate than simulation models based on social science theories. Some engineering projects would not have been possible without computers. The Apollo project, which sent man to the moon, could not be completed without computers.