In order to adequately cover the response surface, you want to use a design that has interior points. By default, Minitab augments a design by adding interior points to the design. Minitab adds axial points and a center point if it is not already in the base design. Each of these additional points is a complete mixture - that is, a mixture in which all components are present at the same time. A design with these interior points would provide information about the inner portion of the response surface and lets you model more complicated curvature.
These points are primarily used to examine the lack-of-fit of a model. In addition, a design with these interior points would provide information about the inner portion of the response surface and lets you model more complicated curvature.
Each axial point is added halfway between a vertex and the center of the design.
The following figures show the points that are added when you augment a second-degree three-component simplex lattice design with both axial points and a center point.
Unaugmented
Augmented
Note
If you do not want to augment your design, when you create the design choose and deselect Augment the design with a center point and/or Augment the design with axial points.
