## Forum: Reliability & Maintainability Questions and Answers## Topic: Reliability & Maintainability Questions and AnswersTopic Posted by: Reliability & Maintainability Forum
(src_forum@alionscience.com
)Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
## Original Message:Posted by: LE GALLIOT
(david.le.galliot@sonovision-itep.fr
)Organization:SONOVISION-ITEP
Date posted: Mon Apr 23 10:58:21 US/Eastern 2007
Subject: Use of level confidence with NPRD-95
Message:Hello, I have a question about use of confidence level with NPRD-95: According to NPRD-91 and 95, "the natural logarithm of the observed failure rates is normally distributed with a sigma=1.5. This indicates that 68% of actual failure rates will be between 0.22 and 4.5 times the mean value". Or, if the parameters of the lognormal distribution are mu and sigma, then the median of the distribution is given by exp(mu) and the mean is given by exp(mu+0.5*sigma*sigma). This indicates, for sigma =1.5, that 56% of actual failure rates will be between 0.2 and 4.5 times the mean value and 68% of actual failure rates will be between 0.2 and 4.5 times the median value. I think that there is an error in NPRD-91 an NPRD-95: either it would be necessary to read “This indicates that 56% of actual failure rates will be between 0.22 and 4.5 times the mean value", either it would be necessary to read “This indicates that 68% of actual failure rates will be between 0.22 and 4.5 times the median value". Is anyone could tell me what really represent all listed failure rates on the both NPRD-91 an NPRD-95: the medians or the means of the lognormal distributions? |