Well, that makes perfect sense. The standard approach is sensible in my experience.
Basically, if you set an availability you also set an MTTR. Depepending on the MTTR, you need a certain MTBF to achieve the availability. You can easily see it, take 90% availability and MTTR=5 mins, respectively 10 mins. This operation will cut the MTBF in half.
Note that the Nex-exp distribution needs only 1 parameter (its mean).
Thank you for your reply!
These are the values I get from changing the availability while having the same MTTR at 60s. The MTBF is the mean parameter in the Negexp distribution for Interval.
My goal here is to use the same calculation in Excel to be able to show users the effect their entered availability has on the MTBF. But I can't seem to get it to work.
Well, I get these results with example values:
|MTTR [min]||Availability||MTBF [min]||= meanvalue of interval-distribution|
I've attached the Excel-file. The basic formula is:
Availability = MTBF / (MTTR + MTBF)
With a little math you can rearrange it:
MTBF = (Availability * MTTR) / (100% - Availability)
And the MTBF is the mean-value of the failure-duration-distribution.
Does this help?