Prior to version 8, center points were not used to help fit a factorial model. Stat-Ease uses all points in the design to fit a polynomial model used for predictions.
This change is not without some consequences. Because the response surface is now being adjusted to fit the center points, the model graphs, point predictions, standard errors and interval estimates will be different than those found in previous versions of the software for factorial designs with center points.
The prediction interval has been improved and moved to the confirmation node.