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Minimum time for systems availability spec?

Minimum time for systems availability spec?

Minimum time for systems availability spec?

(OP)
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"?

RE: Minimum time for systems availability spec?

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)

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