Augmenting the Design
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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 simultaneously present. A design with these interior points would provide information on the inner portion of the response surface and allow you to 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 on the inner portion of the response surface and allow you to model more complicated curvature.

Each axial point is added halfway between a vertex and the center of the design. For calculations of these additional points, see Placement of axial points in augmented designs.

The illustrations below 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.

To compare some other three-component designs, see the table under Choosing a Design. To view any design in Minitab, use Simplex Design Plot.