In this issue:

Issue 12, February 2002

 Hot Topics Reliability Basics Tool Tips Hot News On the Hot Seat Contribute to HotWire

Reliability HotWire is a service of ReliaSoft Corporation

To subscribe or update your e-mail preferences, go to http://www.weibull.com/hotwire/index.htm

Hot Topics
Multimodal Analysis
In life data analysis, the goal is often to fit a good model to the data set. Once a good model has been fitted to the data, other analyses and predictions can be made. Life data can be represented visually on a probability plot. Ideally, the data points on the probability plot should fall in a straight line, indicating a good fit to the specific distribution. If the data points do not fall in a straight line, this is an indication that the chosen distribution is not a good choice to model the data, and another distribution should be chosen. One of the potential reasons for the inability to fit a straight line to the data is the presence of multiple failure modes. Multimodal data sets can often be recognized by the pattern evident on the probability plot. If the points on the plot appear to have a distinct "dogleg," or seem as if they could be modeled by two straight lines of different slopes, this may be indicative of a multimodal data set. In this article, we take a look at methods for analyzing multimodal data sets.

Reliability Basics
The purpose of this section is to offer background and advice to the novice reliability engineer.

Defining Distributions - Part 1: The Probability Density Function
Generally speaking, the object of performing a life data analysis is to be able to predict the future performance of a product. A number of units are placed on test and run until failures occur. The time-to-failure data set is then analyzed and modeled, and predictions regarding the product's failure behavior are calculated. These are the familiar life data analysis metrics such as reliability, failure rate, etc. The basis for all of these metrics is a mathematical function that models how the failure occurrences are distributed over time. This function is called the probability density function or pdf. It is the basis for almost all of the reliability metrics of interest. In this article, we take a look at how the pdf is formulated.

In next month's Reliability Basics, we will look at how the pdf function is used to develop other commonly used functions.

Tool Tips

• How can I get analytical values such as failure rates for the system analysis in BlockSim?

• How can I calculate Weibull MLE parameter estimates for data sets with very small variance in Weibull++?

Hot News
 Advanced accelerated testing analysis training available with "QALT Boot Camp" - ReliaSoft will offer another intensive three-day course on advanced topics, concepts and applications of quantitative accelerated life testing (QALT) with its "QALT Boot Camp."  This is a three-day seminar that covers advanced topics in accelerated test data analysis such as time-varying stress and multiple stress testing. The next QALT Boot Camp will be held in Tucson, June 26-28, 2002. For more information on course content and how to register, see http://www.ReliaSoft.com/seminars/qalt.htm. Next Seminar in Tucson, February 18 - 22  - The next ReliaSoft "Master the Subject, Master the Tools" seminar will be held in Tucson from February 18-22, 2002 at the Sheraton Hotel and Suites. Topics for this five-day seminar include life data analysis, accelerated life testing and system reliability, maintainability and availability. For more information on course content and how to register, see http://www.ReliaSoft.com/seminars/master.htm. ReliaSoft at RAMS - ReliaSoft would like to thank those of you who stopped by the ReliaSoft booth in the vendors area during the recent 48th Annual Reliability and Maintainability Symposium (RAMS) in Seattle, Washington. ReliaSoft personnel presented papers on FRACAS systems, online quality tracking and accelerated testing. We hope we helped contribute to making this a productive and interesting symposium for you.
On The Hot Seat
Once again, we present the feature "On The Hot Seat," where reliability professionals share their experiences with ReliaSoft applications, and relate how they have improved their day-to-day job assignments.

This month "on the hot seat" is Brian Haan, who is the reliability manager for LaserComm Inc. in Plano, Texas. In addition to developing and managing the reliability program, he is responsible for identifying means to raise device reliability beyond current industrial standards.