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03-15-2017 03:32 AM

Hello,

Could anyone advice me how the mean value of the Negexp-distribution for the failure interval is calculated based on Availability and MTTR? I've seen at 50% availability, the mean value of the interval is the same as the entered MTTR.

Thank you in advance!

Solved! Go to Solution.

4 REPLIES

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03-15-2017 04:02 AM

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).

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03-15-2017 05:53 AM

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.

Availability, MTBF

10, 6.6667

20, 15

30, 25.7143

40, 40

**50, 60**

60, 90

70, 140

80, 240

90, 540

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.

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03-15-2017 06:07 AM

Well, I get these results with example values:

MTTR [min] | Availability | MTBF [min] | = meanvalue of interval-distribution |

10 | 95% | 190 | |

10 | 90% | 90 | |

10 | 80% | 40 | |

10 | 50% | 10 | |

10 | 20% | 2.5 | |

20 | 95% | 380 | |

20 | 90% | 180 | |

20 | 80% | 80 | |

20 | 50% | 20 | |

20 | 20% | 5 |

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?

Regards,

Alex

____________________________________________________________

Alex Dilg, Consultant at**Sim**Plan AG (www.SimPlan.de)

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Alex Dilg, Consultant at

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03-15-2017 06:14 AM

Yes, that was exactly what I needed, thank you!

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