Minimum time for systems availability spec?

[VTgrad]

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"?

 

[IRstuff]

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