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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 Rowland (darowland@ieee.org )
Date posted: Mon Jan 5 14:49:48 US/Eastern 2004
Subject: System Reliability Analysis
Message:
I have a system that was originally placed in service in late 70's. Computerized maintenance records are not available until June 1991. Also, the records are not typically detailed enough to bin by failure mode. Essentially, the statistic I have available is "time of repair request". I have considered cumulative time to repair request with t0 = June 1, 1991. Using truncated portions of example data from MIL-HDBK-189, I find this approach unacceptable. I also find that goodness of fit tests of my actual data typically fail with this approach. The other approach I have considered is time between repair requests, since these times would be independent of actual system age. This would, then, allow me to determine the recurrence rate. I could use this recurrence rate in subsequent modeling with the understanding that times must be accurately specified. Is there an optimal methodology for handling truncated system repair data such as mine? Thanks.


Reply:

Subject: System Reliability Analysis
Reply Posted by: swami (swami.nathan@sun.com )
Date Posted: Mon Jan 5 17:53:00 US/Eastern 2004
Message:
Left censored data is fairly common in these systems. you could try a simple non-parametric approach like plot cumulative repair counts vs age or date. Then simply take the slope of that line to get the rate of occurrence of repairs. If the slope curve is fairly flat then you have a constant failure rate if not increasing or decreasing. slope calculation can be done by numerical differentiation i.e., take 5 points and calculate a slope, plot it on the midpoint, and so on. can be easily done in staroffice or excel.


Reply:

Subject: Reliability with missing data
Reply Posted by: Larry George (pstlarry@comcast.net )
Organization: Problem Solving Tools
Date Posted: Thu Jan 8 20:06:41 US/Eastern 2004
Message:
Do you know your installed base; i.e., number of units in service and approximately when they were placed in service? Counts by quarter or year are OK; estimates are OK. If so, send installed base counts and repair times or counts to pstlarry@comcast.net, and I will send back nonparametric estimates of age-specific field reliability (distribution of ages between repairs) and repair rate functions, free of charge. The estimators are max. likelihood and least squares. See http://www.fieldreliability.com/RenewalKM.ppt for the max. likelihood description.


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