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 |
linear and two-way interactions |
A B C D |
full quadratic (default) |
A B C D |