What factor values should I use in a response surface design?

In a response surface design, you designate a low level and a high level for each factor. These factor levels define the proportions of the "cube" around which the design is built. The "cube" is often centered around the current operating conditions for the process. For a central composite design, you may have design points in the "cube," on the "cube," or outside the "cube." For a Box-Behnken design, the factor levels are the lowest and highest points in the design.

By default, Minitab sets the low level of all factors to -1 and high level to +1.

Note

In a central composite design, the values you enter for the factor levels are usually not the minimum and maximum values in the design. They are the low and high settings for the "cube" portion of the design. The axial points are usually outside the "cube" (unless you specify an α that is less than or equal to 1). If you are not careful, this could lead to axial points that are not in the region of interest or may be impossible to do.

Ensure that all design points are between the minimum and maximum factor levels

For Central Composite Designs, choosing Axial points in the Factors sub-dialog box guarantees all the design points will fall between the defined minimum and maximum value for the factors. Minitab will then determine the appropriate low and high settings for the "cube" as follows:
  • Low level setting = ((α – 1) maximum + (α + 1) minimum) / (2*α)
  • High level setting = ((α – 1) minimum + (α + 1) maximum) / (2*α)

Change the factor levels of an existing design

To change the factor levels after you have created the design, choose Stat > DOE > Modify Design > Modify Factors and enter the new information.

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