Model Browser User's Guide

Prediction Error Variance Viewer

You can use the Prediction Error Variance (PEV) viewer to examine the quality of the model predictions. You can examine the properties of designs or global models. When you open it from the Design Editor, you can see how well the underlying model predicts over the design region. When you open it from a global model, you can view how well the current global model predicts. A low PEV (tending to zero) means that good predictions are obtained at that point.

The PEV viewer is only available for linear models and radial basis functions.

When designs are rank deficient the PEV Viewer appears but is empty; that is, the PEV values cannot be evaluated because there are not enough points to fit the model.

• From the Design Editor, select Tools -> PEV Viewer.
• From the global level of the Model Browser, if the selected global model is linear or a radial basis function:
• You can click the  toolbar button to open the PEV viewer.
• Alternatively, you can select Model -> Utilities -> Prediction Error Surface.

  

The default view is a 3-D plot of the PEV surface.

The plot shows where the model predictions are best. This example shows an MBT model response feature. The model predicts well where the PEV values are lowest.

Display Options

• The Surface menu has many options to change the look of the plots.
• You can change the factors displayed in the 2-D and 3-D plots. The drop-down menus below the plot select the factors, while the unselected factors are held constant. You can change the values of the unselected factors using the buttons or edit boxes in the frame, top left.
• The Movie option shows a sequence of surface plots as a third input factor's value is changed. You can change the factors, replay, and change the frame rate.
• You can change the number, position, and color of the contours on the contour plot with the Contours button. See the contour plot section (in the Response Surface view of model selection and evaluation) for a description of the controls.
• You can select the Clip Plot check box, as shown in the preceding example. Areas that move above the value in the Clipping envelope edit box are removed. You can enter the value for the clipping envelope. The edges of the clip are very jagged; you can make it smoother by increasing the numbers of points plotted for each factor (enter values in the Pts edit boxes in the top frame).

When you use the PEV viewer to see design properties, optimality values for the design appear in the Optimality criteria frame.

Note that you can choose Prediction Error shading in the Response Feature view (in Model Selection or Model Evaluation). This shades the model surface according to Prediction Error values (sqrt(PEV)). This is not the same as the PEV viewer, which shows the shape of a surface defined by the PEV values. See Response Surface View.

Optimality Criteria

No optimality values appear in the Optimality criteria frame until you click Calculate. Clicking Calculate opens the Optimality Calculations dialog. Here iterations of the optimization process are displayed.

In the Optimality criteria frame in the PEV viewer are listed the values of the input factors at the point of maximum PEV (Gmax). This is the point where the G optimality value is calculated. The D and V values are calculated for the entire design, not just at the point Gmax.

For statistical information about how PEV is calculated, see Prediction Error Variance and Prediction Error Variance for Two-Stage Models in the Technical Documents section.

Importing Constraints

Technical Documents