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A fundamental analysis of the meaning of second-moment reliability in multivariate problems is presented. The format described is entirely derived from one basic assumption concerning the measurement of reliability. All formulations are exact, and approximations involving the assumption of small variance are only introduced to simplify practical equations. The format is fully invariant under any change of formulation of the failure criteria consistent with the laws of algebra and mechanics.
Hasofer et al. (1974) studied this question.