Reliability HotWire

Issue 88, June 2008

Tool Tips

* How can I tell if a transformation is required for my response in DOE++ and, if so, which one I should use?
The Residual Probability plot is a simple way to evaluate the fit of the model in DOE++. If all points fall more or less on the line, as shown next, the model is good.

If a noticeable pattern is present in the plot, as shown next, it may be appropriate to apply a transformation to the response.

The Box-Cox Transformation plot can help determine what transformation, if any, should be applied. The plot shows the sum of squares of the residuals plotted against lambda. The value of lambda at the minimum point of this curve is considered the "best value" of lambda, and indicates the appropriate transformation, which is also noted by default in the lower title of the plot.


[Click to Enlarge]

To apply a transformation to the response, click the Transformation icon in the Design tab or Analysis tab Control Panel.

On the General page of the Transformation window, select the response you want to work with from the Response drop-down menu. (Note that when the Transformation window is accessed from the Analysis tab Control Panel, the choice you make will be applied only to the response currently selected in the Control Panel. The Response drop-down menu will be disabled.) You can then select a transformation from the Transformation drop-down menu. The equation for the transformation will be displayed at the bottom of the window and text fields for any required inputs will be available in the Parameters area, as shown next.

Note that the information displayed in the Box-Cox Transformation plot is also available on the Box-Cox Transformation page of the Transformation window. Click the Calculate button in the Box-Cox Transformation Results area. The Box-Cox Transformation Results area will then display the best lambda value for the response, including the two-sided confidence bounds at the level determined by the risk level (alpha) you have specified, which helps in selecting an appropriate transformation. The recommended transformation, if any, will be displayed in the Recommended Transformation area, along with any recommended parameters, as shown next.

You can then return to the General page of the Transformation window to implement the recommended transformation.

* Why is "Pnz" sometimes displayed in the results area of the Standard Folio Control Panel in Weibull++?
Pnz stands for "percent non-zero." When your data set contains failures at time 0, this figure is automatically displayed in the results provided by Weibull++.

Percent non-zero is calculated as follows:

Pnz = 1 - (Number of failures at time 0) / (Number of all entries – Number of suspensions at time 0)

The suspensions at time 0 are ignored and do not count in the analysis.

Percent non-zero is also displayed by default in the lower title of plots, along with the calculated parameters. It is denoted by Z.

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