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An analysis is presented of the effect of partial failure modes on the calculation of an optimum amount of redundancy. (Partial failure modes are modes that are catastrophic to the part but not to the system.) The analysis is limited to systems that are not repaired, whose performance can be measured with a one-dimensional capacity index, and that are composed of parts that fail in only one mode. The problem is formulated in terms of loss coefficients and state probabilities and a model is given to organize computation of the state probabilities. The analysis is incorporated into a redundancy optimization program so that, for the first time, optimal redundancy can be calculated for systems with partial failure modes. This program is applied to an example system and the results are explained. It is shown that the partial failure mode analysis will always yield a lower expected loss, will thus require less redundancy, and hence will yield a lower initial system cost than the equivalent total failure-mode analysis.