Validation of models

This paper shows which statistical techniques can be used to validate simulation models, depending on which real-life data are available. Concerning this availability, three situations are distinguished (i) no data, (ii) only output data, and (iii) both input and output data. In case (i) -no real data -the analysts can still experiment with the simulation model to obtain simulated data; such an experiment should be guided by the statistical theory on the design of experiments. In case (ii)only output data -real and simulated output data can be compared through the well-known two-sample Student t statistic or certain other statistics. In case (iii) -input and output data -trace-driven simulation becomes possible, but validation should not proceed in the popular way (make a scatter plot with real and simulated outputs, fit a line, and test whether that line has unit slope and passes through the origin); alternative regression and bootstrap procedures are presented. Several case studies are summarized, to illustrate the three types of situations.