You can fit a regression with life data model with the following terms:

- Factors
- Covariates
- Crossed or nested factors
- Covariates that are crossed with each other or with factors, or nested within factors

To enter the model terms in Model on the main dialog box, use the following rules:

- Use * to specify an interaction term. For example, A*B is the interaction of factors A and B.
- Use ( ) to indicate nesting. When B is nested within A, type B(A). When C is nested within both A and B, type C(A B). Terms in parentheses are always factors in the model and are listed with blank spaces between them.
- Use | or ! to indicate that you want to include all factors and their interaction terms in the model. For example, A | B | C will include A, B, C, A*B, A*C, B*C, A*B*C in the model.
- Use – to remove terms from the model. For example, A | B | C – A*B*C includes all factors and their interactions except for A*B*C.
- Models must be hierarchical. In a hierarchical model, if a higher-order term is included, all lower order interactions and main effects that comprise the higher-order term appear in the model. For example, a model that includes the interaction term A*B*C is hierarchical if the model includes the following terms: A, B, C, A*B, A*C, and B*C. Alternatively, A|B|C - A*B is not hierarchical. For more information, go to Restrictions on models for regression with life data.

For example, suppose that A is a factor and X is a covariate in a regression with life model. The following examples show the notation that you can use to enter different terms for the model.

- A X A*X
- Fits a full model with a covariate crossed with a factor
- A | X
- An alternative way to specify a full model with a covariate crossed with a factor
- A X X*X
- Fits a model with a covariate crossed with itself, creating a squared term
- A X(A)
- Fits a model with a covariate nested within a factor

When you specify the model terms and the column names contain spaces or special symbols, enclose the column name in single quotation marks. For example, if the columns names are Brown sugar and White sugar, specify the terms as follows: 'Brown sugar' 'White Sugar' 'Brown sugar' * 'White Sugar'.