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Measuring Dependability as a Mean Failure Cost

The Mean Time to Failure (MTTF) is a commonly accepted measure for system reliability. Some variations of it (MTTD, MTTE) have also been adopted to measure other dependability attributes [1]. In this short note we wish to submit a tentative challenge to this measure, propose an alternative, and discuss how the alternative can be deployed. When we say that a system has an MTTF , we mean that the mean time to the failure of the system with respect to some implicit specification is . In doing so, we are usually making two implicit assumptions: