State reduction for semi-Markov reliability models

Trimming, a method for reducing the number of states in a semi-Markov reliability model, is described, and an error bound is derived. The error bound uses only three parameters from the semi-Markov model-the maximum sum of rates for failure transitions leaving any state, the maximum average holding time for a recovery-mode state, and the operating time for the system. The error bound can be computed before any model generation takes places which means the modeler can decide immediately whether the model can be trimmed. The trimming has a precise and simple description and thus can be easily included in a program that generates reliability models. The simplest version of the error bound for trimming is presented. More accurate versions can be obtained by requesting more information about the system being modeled