The number of center points, along with α (for a central composite design), determines whether or not a design can include orthogonal blocks. By default, Minitab chooses the number of center points to include orthogonal blocks.
The inclusion of center points provides an estimate of experimental error and lets you verify the adequacy of the model. Verifying the adequacy of the fitted model is important because an incorrect model can produce misleading conclusions.
The default number of center points is shown in thedialog box.
When a Box-Behnken design includes blocks, the center points are divided equally (as much as possible) between the blocks.