For every project anew, simulation practitioners have to specify the design features of a simulation model. At issue are: which type of perfonnance measures to use, which way to validate the model, and, of course, the question if a steady-state simulation is appropriate in order to generate useful results. In resolving these issues, however, practitioners are left alone: with the exception of some casuistic approaches (BratleylFoxi Schrage 1987 , Hoover/Perry 1989 , LawlKelton 1991, there is no

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... encompassing framework available which supports the classification of a system to be simulated. Our argument is backed by two case studies. We present two real-life systems-a clinical laboratory and a data communication network-that show rather similar characteristics as to the life cycles of entities and the daily workloads. However, these two systems require rather different modelling approaches. While a steady-state approach lends itself to simulate the clinical laboratory, it does not in the case of the data network. Thus, a closer scrutiny shows that also practical considerations have to be incorporated into a classification scheme that establishes the link between the real-world system and the design issues. Drawing on these experiences we propose a structured process that helps to identify the appropriate type of simulation model to be implemented. This structured process can be reduced to a three-dimensional framework that takes into account the following criteria: the stationarity properties of the simulated system, the time horizon of the real-life system compared to the time horizon of the simulation runs, and the research purpose. This framework not only allows to meaningfully categorize simulation problems but also to assign the appropriate methods for tackling the start-up problem, collecting output data, implementing variancereduction techniques, and validating the simulation model (cf. Liebl 1992). References: Bratley, P.; Fox, B. L.; Schrage, L. E.: A Guide to Simulation (Second Edition); New York 1987 Hoover, S. V.; Perry, R.