To strengthen the current Box-Behnken design near the center region:
Close the Design Evaluation Tool.
Return to the Design Editor window.
Select Design -> Classical -> Box-Behnken.
Click OK to replace the current points with a new design.
Increase the number of center points and click OK.
Once again select Tools -> PEV Viewer and review the plots of prediction error variance and the new values of optimality criteria in the optimality frame (bottom left).
Review the leverage values of the center points. From the Design Editor window, use Tools -> Evaluate Design and go to Hat Matrix.
Try other designs from the Classical Design Browser. Compare Full Factorial with Central Composite designs; try different options and use the PEV viewer to choose the best design.
Note
You cannot use the PEV viewer if there are insufficient points in the design to fit the model. For example, you cannot fit a quadratic with less than three points, so the default Full Factorial design, with two levels for each factor, must be changed to three levels for every factor before you can use the PEV viewer.
When you are satisfied, return to the Design Editor window and choose Edit -> Select as Best. You will see that this design node is now highlighted in blue in the tree. This can be applied to any design.
When you are creating designs before you start modeling, the design that you select as best is the one used to collect data.
