Model Browser User's Guide

Prediction Error Variance Viewer

A useful measure of the quality of a design is its prediction error variance (PEV). The PEV hypersurface is an indicator of how capable the design is in estimating the response in the underlying model. A bad design is either not able to fit the chosen model or is very poor at predicting the response. The PEV Viewer is only available for linear models. The PEV Viewer is not available when designs are rank deficient; that is, they do not contain enough points to fit the model. Optimal designs attempt to minimize the average PEV over the design region.

Select Tools -> PEV Viewer.

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

This shows where the response predictions are best. This example optimal design predicts well in the center and the middle of the faces (one factor high and the other midrange), but in the corners the model has the highest error. Even in the corners the errors are small; looking at the scale, PEV values under 0.25 are not large.

Try the other 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 pop-up 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 and edit boxes in the Input factors list, 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, as shown.

  

Design Displays

Improving the Design