Reliability HotWire

Issue 81, November 2007

Tool Tips

* How can you analyze data sets with more than one failure mode in Weibull++?
Weibull++ offers two options for analysis when a group of products fails due to more than one failure mode.

  • Competing Failure Modes: For data with up to four failure modes, you can enter all of the data in a single Data Sheet, using subset IDs to differentiate among the modes. Select a Weibull, exponential, normal or lognormal distribution with the CFM option under Parameters/Type. When you click Calculate, the Competing Failure Modes Select Subsets window will appear, as shown next.

This window allows you to specify which subset ID(s) represent which failure modes. In the analysis, each failure mode is analyzed separately, with data from the other modes treated as suspensions. The failure modes are then combined in a series reliability model, wherein the overall reliability of the series system is equal to the product of the reliability values of the failure modes that compose the system.

  • Reliability Block Diagrams: Weibull++'s Diagrams offer greater flexibility for analyzing more complex systems of failure modes. You can consider unlimited failure modes, each analyzed with the lifetime distribution appropriate to its data set, and model the failure behavior by describing the reliability-wise relationships among the modes (i.e. series, parallel, k-out-of-n). The data set for each failure mode is entered on a separate Data Sheet and analyzed with the chosen distribution. When you create the Diagram, a template for each Standard Folio, containing a block for each Data Sheet within the Folio, is created. You can drag these blocks into the Diagram and arrange and connect them using node blocks and connector blocks to model the configuration of the system.

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* How can you be sure your software is up to date?
ReliaSoft uses minor version "service releases" to add features and resolve any issues with the software. These updates are free to registered users. We encourage you to check the Web site occasionally to make sure that you are always running the most up-to-date version of the software that is available. A full list of the latest versions of all of ReliaSoft's active products is located at

Not sure which version you're running? Simply select About from the Help menu of the software. You'll see the version number displayed in the window, as well as in the title bar.

If you would like to receive notifications when new versions are released, you can visit to subscribe to ReliaSoft's Recent Product Releases RSS Feed.

Copyright 2007 ReliaSoft Corporation, ALL RIGHTS RESERVED