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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Posted by: Andrew Comons
Date posted: Fri Mar 29 13:59:26 US/Eastern 2002
Subject: Exponential Conditional Reliability...A Paradox?
I recently came across a derivation of the "Exponential Conditional Reliability" equation which states that regardless of previous accumulated age, the reliability of a system is only dependent on the duration of the present mission, i.e., no "memory" of what occurred prior to the present mission. I fully understand the arithmetic of the derivation, but not the principle. According to the theory, I could momentarily shut a system down in the middle of a mission, turn it back on and expect the same probability of success as at the start of the initial mission. This does not seem practical or realistic, but the math says oterwise. Can anyone explain this paradox?? Thanks in advance

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Subject: Exponential Failure Rate
Reply Posted by: Donald L. Meaker ( )Northrop Grumman, Integrated Systems
Date Posted: Fri Jun 14 9:39:00 US/Eastern 2002
There is a difference in using the Exponential Distribution as the really true distribution and as an approximation. If you are performing Weibull Analysis, and have filtered your data so that you are analyzing all the same failure mode, and then your analysis indicates that the shape parameter is 1, (ie, an exponential distribution) I would check the stress level to see if there was a voltage spike or a bump on my course that was the cause of failure. On the other hand, If I am using the exponential distribution as an approximation of the failure rate for a complex component, modeling several different failure modes, (Dreineck's Therom) then no over stress is necessarily present.

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