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Minimum time for systems availability spec?

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VTgrad

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When writing a requirement for system availability, is there a minimum time period that should be specified for statistical significance?
For example, "The ABC system shall provide 99.95% availability, per quarter." Is "per quarter" too short? Should this be "one year"?
 
Availability is usually calculated from the MTBF, so if you've already expressed the MTBF, then there's no need to caveat the availability.

A = MTBF/(MTBF+MTTR+MLDT)

TTFN

FAQ731-376
 
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