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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: Arthur Fries ( )
Organization:Institute for Defense Analyses
Date posted: Tue Aug 24 13:23:01 US/Eastern 1999
Subject: Reliability Surveillance Testing
All: Often systems are "stockpiled" awaiting use in an operational environment (e.g., missiles or chemical suits stored until wartime), and there is a need to regularly (e.g., yearly) inspect the stockpile to check for possible reliability degradation. I am interested in methodologies for determining (1) how many articles to test at each prespecified time point, and (2) how to combine testing results to estimate current and future system capabilities. I am particularly interested in methodolgies that account for trends in the data, e.g., recent testing failures might reasonably be expected to signal additional testing requirements. Thanks in advance for your consideration. Arthur Fries


Subject: Reliability Surveillance Testing
Reply Posted by: Bruce Dudley ( )
Organization: Reliability Analysis
Date Posted: Tue Aug 31 9:45:15 US/Eastern 1999
Determining the number of test samples should be a function of the confidence level and defect rate expected. LTPD(Lot Tolerance Percent Defective) sample sizes are contained in MIL-PRF-19500k Performance Specification Semiconductor Devices. For example, a 95% confidence for a 5% defect rate would require 58 test unit without failure. With regard to testing at prescribed time points, you should review the Hughes Aircraft study of dormant missiles which indicates that less testing is better as more failures are introduced from the test equipment/test operators than occur from the storage conditions. This report is called: "Dormant Missile Test Effectiveness" RADC-TR-89-276.

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