This paper presents a systematic approach for computing metrics and performance indices of interdependent critical infrastructures based on their information content, expert views and risk analysis capabilities. The paper also proposes a risk-based methodology that aims to monitor interdependent services based on generic risks and assurance levels using security properties: confidentiality, integrity and availability. Such approach helps to determine the quality of the provided service and allows each CI provider to react and adopt the best behavior corresponding to the security status and assurance level of its different services. A real life example is used to illustrate the computation of the measures and performance indices, and their application to the analysis of critical infrastructure interdependencies