It is commonly agreed between modeling methodologists that model validation is one of the most important stages in the model building process. Many papers addressing this subject have been published and an SCS Technical Committee on Model Credibility has been established in order to generalize and summarize the experiences in this field (see Appendix) . However, at the present stage of research there are almost no suggestions concerning concrete methods of validation. Practically all authors only discuss definition of validation - not methods. The number of papers dealing with methods of model validation is also rather limited.
The reason for this gap between methodological consciousness and the practice of model building seem to be obvious - the discussion stays at too high a level of abstraction. In general, all authors consider "model" as a description of reality, and on this level of concretization it is only possible to generate rather general statements, frequently true but without operational meaning. The author of this paper believes that, in order to examine validation methods, it is necessary to specify more precisely the model under consideration, the properties of the model, the modeling techniques, and, most importantly, the purpose of the model.
The aim of this paper, therefore, is to present a classification of models and an analysis of the modeling process from the point of view of model validation. At this stage of the investigation, however, it is not yet possible to design, nor to analyze, methods of validation. Our goal is to design a framework for model validation as a first and important step in establishing a model validation methodology
