Issue 1, March 2001

Why doesn't the calculated MLE line track the data points on my probability plot in Weibull++? 

The reason that the fitted MLE model does not track the data points is due to differences in the way the plot and the MLE model are generated. The probability plot incorporates estimates of the unreliability in order to provide y-axis values to go with the failure times on the x-axis. In Weibull++, these unreliability estimates are provided by median ranks or Kaplan-Meier estimates. The maximum likelihood estimator (MLE) method of parameter estimation only uses the failure and suspension (x-axis) data when generating the unreliability model. Consequently, the fitted model may not necessarily track the data points on the plot. However, this does not mean that the model or the plot is "wrong," just that they were generated using two different methods.

This is most likely due to another application you may have had running when you installed or upgraded the application. When installing or upgrading any ReliaSoft application, make sure you have closed or disabled any other applications. This includes applications that may be running in the background, such as virus scan software.

In order to change the values in more than one block, highlight the collection of blocks of interest. This can be accomplished either by dragging a box around the group of blocks you want to change, or by clicking the individual blocks while simultaneously holding down the Ctrl key. Double-click the highlighted blocks to begin editing (a message box will pop up to inform you that you are changing the values of more than one block). Note that once the changes have been entered, the blocks will have the same values for all parameters, even parameters that were not changed during the editing process.

Weibull++ and ALTA have a built-in graphics editing application called RS Draw. RS Draw can be accessed by selecting the Edit Plot option from the Plot Options menu, or by clicking the RS Draw icon on the toolbar:

RS Draw allows the user to edit and annotate plots generated in Weibull++ and ALTA. These plots can then be saved in the Windows metafile format (*.wmf), which can be imported or pasted into other applications such as Microsoft Word or PowerPoint.

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Last updated: 07/17/2008 08:46:18 PM

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