7.5.3 Misuses of Simulation

By "disadvantages" of simulation, we mean difficulties that are an inherent part of the technique. By "misuses" we mean problems that result from the naive or erroneous or-in the worst case-even dishonest uses of simulation. We discuss such misuses below.

By far the most common is the use of simulation as an expensive way of obtaining obvious or easily derivable results. The coauthors of this book have encountered numerous instances where large-scale simulation models have been developed and are being used (often at great cost in money10 and manpower) to provide information that is readily obtainable otherwise, through some simple mathematical expression. The tendency for such misuse is particularly pronounced within government agencies, because of the fact that it is often easier in such cases to obtain a budgetary appropriation for computer-related expenditures (including computer programmers and "systems analysts") than for personnel with analytical competence and skills.

A second, potentially more damaging, misuse of simulation results from The fact that simulation packages are often used without any understanding of their logic and are mysterious "blackboxes" to all but the designers of the package (if that). For urban service systems, such packages may run into many hundreds, perhaps even thousands, of lines of computer code. Thus, it is practically impossible even for an expert to check fully the logic and assumpions of the simulation program, without assistance from its designer. As a result, a simulation may contain hidden (intentionally or unintentionally) assumptions which are unrealistic or only partly valid without its users being aware of the fact. The only way to avoid such unfortunate situations is by setting forth strict documentation requirements for the simulation's logic and for the program code (see Chapter 8), coupled with a continuing relationship between program developer and program user.

Finally, simulations can be misused by placing too much faith in their results. This is a problem that usually afflicts simulation users rather than hose who develop the programs. The word "simulation" carries a conotation of "exact duplication" and "mimicking." It is often forgotten hat simulations, despite including myriad details, are based on idealized models of reality that include many simplifying assumptions and approximations. Not unlike analytical models, it is the responsibility and obligation of the designer of a simulation program to warn its users constantly about he limitations of the simulation and thus prevent such misuse.

10The cost of using simulation models in such a thoughtless manner is often exacerbated by the temptation to "include everything" (the "kitchen sink") in a simulation, as noted in Chapter 8.