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Complex real-time systems need to address dependability requirements early on in the development process. This paper presents a model-based approach that allows developers to analyze the dependability of use cases and to discover more reliable and safe ways of designing the interactions with the system and the environment. We use a probabilistic extension of statecharts to model the system requirements. The model is then evaluated analytically based on the success and failure probabilities of events. The analysis may lead to further refinement of the use cases by introducing detection and recovery measures to ensure dependable system interaction. A visual modelling environment for our extended statecharts formalism supporting automatic probability analysis has been implemented in AToM3, A Tool for Multi-formalism and Meta-Modelling. Our approach is illustrated with an elevator control system case study. [ABSTRACT FROM AUTHOR]