While there is a growing interest for component-based systems in industry, little effort has so far been devoted to quality evaluation of these systems. This paper presents the definition of measures for two quality factors, namely robustness and "diagnosability" for the special case of OO systems for which the approach known as Design by Contract has been used. The main steps in constructing these measures are given, from informal definitions of the factors to be measured to the mathematical model of the measures. To fix the parameters, experimental studies have been conducted, essentially based on applying mutation analysis in the OO context. Several measures are presented that reveal and estimate the contribution of contracts quality and density to the overall quality of a system in terms of robustness and "diagnosability".