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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

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

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Original Message:

Posted by: Andrew Comons
Date posted: Fri Mar 29 13:59:26 US/Eastern 2002
Subject: Exponential Conditional Reliability...A Paradox?
Message:
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


Reply:

Subject: Exponential Paradox
Reply Posted by: Patrick Hetherington (phetherington@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Tue Apr 2 8:40:07 US/Eastern 2002
Message:
The exponential failure distribution is predicated on a random pattern where by the intensity of failures (number or probability) is directly related to the amount of exposure rather than accumulated damage. Items that fail exponentially are generally the result of random overstress. Think of the probability of being hit by lightning (a random overstress situation). The likelihood of getting hit over a ten year period (long period of exposure) is much greater than being hit today (short period). Given that a person was not hit by lightning in the last ten years, however, does not change the probability that they will or will not be hit today.


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