The goal of this study is the development of a probabilistic model for the evaluation of flexibility of an object-oriented design. In particular, the model estimates the probability that a certain class of the system gets affected when new functionality is added or when existing functionality is modified. It is obvious that when a system exhibits a large sensitivity to changes, the corresponding design quality is questionable. Useful conclusions can be drawn from this model regarding the comparative evaluation of two or more object-oriented systems or even the assessment of several generations of the same system, in order to determine whether or not good design principles have been applied. The proposed model has been implemented in a Java program that can automatically analyze the class diagram of a given system.