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Computation of steady-state series-system availability depends on specific assumptions made about the nonfailed components during the system failure. Sherwin recently made the case that the steady-state availability in a series-system is not calculated using the product rule. This commentary offers a clarification by illustrating 2 cases of steady-state series-system availability that frequently arise in reliability engineering. The product rule is valid for steady-state series-system availability under the circumstances: nonfailed components (viz, electrical components) continue to age "normally" during the repair of the failed component. Therefore, in computing steady-state availability of series-systems, it is important for practitioners to determine whether the nonfailed components continue to age "normally" (case-1) or do not age (case-2). It is also shown that for n/spl ges/2, case-1 steady-state availability of the series-system is smaller than case-2 steady-state series-system availability, and is the same for n=1.