1. A method for determining a probability of occurrence that an overall system fulfills a predefined system performance, wherein the overall system is composed of instances of different classes of partial systems and wherein each class of partial system has an associated class fault tree, the method comprising the steps of: (a) Calculating an overall performance for each failure combination of non-failed instances within a class; (b) Automatic selection of failure combinations, the calculated overall performance of which fulfills the predefined system performance; (c) Automatic generation of an overall fault tree based on the class fault trees of partial systems as a function of the selected failure combinations; and (d) Evaluation of the generated overall fault tree for calculating the probability of occurrence of a fulfillment of the predefined system performance by the overall system.