Abstract
The usual criterion of choice between preventive and contingency maintenance, availability, is restrictive in the sense that improvement is, even in ideal circumstances, at most of the order of 10%. Economic criteria are shown to obtain optima at values that do not maximize availability but show better percentage increases in net revenue rate or percentage decreases in maintenance cost. A decision model, valid for all types of life to failure distribution is given and demonstrated by a graph for the Weibull distribution. Finally, if life to failure has the Weibull distribution an easily calculated approximate optimal maintenance schedule is derived and shown to be within predictable limits of the true optima.

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