Selecting Model Terms (Response Surface Design)
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The model you choose determines what terms are fit and whether or not you can model linear or curvilinear aspects of the response surface. If you include any second-order terms (squares or interactions), you can model curvilinear data.

You can fit a linear, linear and squares, linear and two-way interactions, or full quadratic (default) model. Or, you can fit a model that is a subset of these terms. The table below shows what terms would be fit for a model with four factors.

This model type

fits these terms

linear

A  B  C  D

linear and squares

A  B  C  D
A*A  B*B  C*C  D*D

linear and two-way interactions

A  B  C  D  
A*B  A*C  A*D  B*C  B*D  C*D

full quadratic (default)

A  B  C  D  
A*A  B*B  C*C  D*D
A*B  A*C  A*D  B*C  B*D  C*D