Changing the number of center points
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The number of center points, along with a (for a central composite design), determines whether or not a design can be orthogonally blocked. By default, Minitab chooses the number of center points to achieve orthogonal blocking.

The inclusion of center points provides an estimate of experimental error and allows you to check the adequacy of the model (lack of fit). Checking the adequacy of the fitted model is important as an incorrect or under-specified model can result in misleading conclusions.

The default number of center points is shown in the Designs subdialog box. For a table showing the default number of center points for all designs, see Summary of central composite designs and Box-Behnken designs.