In this issue: Issue 106, December 2009
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Hot Topics
Reliability Demonstration Tests for Repairable Systems
Reliability demonstration tests for non-repairable systems are well developed and regularly used by reliability engineers. A tool called the Design of Reliability Tests (DRT) that helps users design such tests is included in Weibull++. For non-repairable systems or components, reliability demonstration tests are used to demonstrate the reliability at a given time with a given confidence level. For repairable systems, users are more interested in demonstrating the MTBF or the number of failures/repairs during a given time period. In RGA 7, a new DRT tool for repairable systems has been designed for this purpose.

Reliability Basics
Comparison of Fisher Matrix and Likelihood Ratio Confidence Bound Methods
In Weibull++, there are several ways to calculate reliability confidence bounds for different distributions. The Fisher matrix (FM) method and the likelihood ratio bounds (LRB) method are both used very often. Both methods are derived from the fact that the parameters estimated are computed using the maximum likelihood estimation (MLE) method. However, they are based on different theories. The MLE estimates are based on large sample normal theory, and are easy to compute. However, when there are only a few failures, the large sample normal theory is not very accurate. Thus, the FM bounds interval could be very different from the true values. The LRB method is based on the Chi-Squared distribution assumption. It is generally better than FM bounds when the sample size is small. In this article, we will compare these two methods for different sample sizes using the Weibull distribution.

Tool Tips
• How can I choose what type of Data Sheet to use in RGA 7?
• What is an easy and affordable way to ensure that I always have the latest version of ReliaSoft’s software?