You can add interaction terms and polynomial terms to your model. By default, the model contains only the factors and covariates that you entered in the main dialog box. Click Default to return to this model at any time.
Terms in the model displays nested factors using parentheses. For example, when Operator is nested in Plant, Minitab displays Operator (Plant).
There are several ways to add terms. We use examples to illustrate them. For the examples, assume that the Factors and covariates list has 3 continuous variables, X, Y, Z and 2 categorical variables, A, B.
- Add terms using selected factors, covariates, and model terms
- Select at least one factor, covariate, or term. To select multiple items or to deselect an item, press the Ctrl key while you click the factor, covariate, or term.
- Interactions through order
- Add all interactions through the specified order. Suppose you select variables X, Y, A and add interactions through order 3. When you click Add, Minitab adds X*Y, X*A, Y*A, X*Y*A.
- Terms through order
- Use to model curvature. This option adds powers and interactions through the specified order. Powers are only for covariates. Suppose you select X, Y, A and terms through order 3. When you click Add, Minitab adds the power terms for X and Y: X*X, Y*Y, X*X*X, Y*Y*Y. Minitab also adds interactions for the predictor variables and powers: X*Y, X*A, Y*A, X*X*Y, X*Y*Y, X*X*A, X*Y*A, Y*Y*A.
- Cross factors, covariates, and terms in the model
- This option can be used in the following ways:
- You can cross two or more factors and covariates. Suppose you select X, Y, Z. When you click Add, Minitab adds the following terms: X*X, X*Y, X*Z.
- You can cross two or more terms that are already in the model. Suppose X*A and X*B are in the model. If you select only these terms and click Add, Minitab adds X*X*A*B.
- You can cross factors and covariates with terms in the model. Suppose X*X and Y*Y are in the model. If you select these terms and factors A, B then click Add, Minitab adds X*X*A, X*X*B, Y*Y*A, Y*Y*B. Each factor is crossed with each model term, but the model terms are not crossed with themselves.
You may need to deselect items so that only the terms you want to cross are selected. To deselect items, press the Ctrl key while you click the factor, covariate, or term.
- Terms in the model
- When you add terms to the model, the terms are listed in the white space in the dialog box. In this white space, you can select individual terms or groups of terms to remove or reorder them.
- Populates the model with only the factors and covariates that you entered in the main dialog box.
- Delete terms
- You can delete one or more terms from the model. Select the terms and click Delete. You can also double-click a term to delete it.
- Reorder terms
- To move a term, select it, then click one of the arrow buttons in the dialog to move the term up or down. You can also move a contiguous block of terms. Click the first term then hold the Shift key and click the last term to select the whole block. Then click the appropriate arrow to move the block.