1. A computer-implemented method for analyzing behavior of a complex system by calculating a model which represents various measured variables as a function of input variables, comprising the following basic steps: selecting various measured points which correspond to different constellations of measured variables and performing measurements to ascertain measured variables on a real system; preparing the model which simulates dependence of the measured variables on the input variables and calibrating the model on the basis of the measured values of the real system obtained at the measured points; subdividing the model into at least two partial models; preparing one first partial model which simulates a first partial set of the measured variables; identifying at least one first main influence parameter having greatest influence on a measured variable simulated by the first partial model; determining an optimum value of each first main influence parameter in each measured point; interpolating each first main influence parameter for all meaningful constellations of input variables to calibrate the first partial model; preparing a further partial model to simulate a further partial set of the measured variables as a function of input variables and the previously ascertained first partial set of the measured variables; identifying at least one further main influence parameter having greatest influence on a measured variable simulated by the further partial model; determining an optimum value of each further main influence parameter in each measured point; interpolating each further main influence parameter for all meaningful constellations of input variables to calibrate the further partial model; and analyzing the behavior of the real system based on the calibrated partial models; wherein said real system is an internal combustion engine and at least one of the following variables are used as the measured variables: inflowing air mass, indexed mean pressure, maximum cylinder pressure, and exhaust gas temperature before catalytic converter and/or turbine.