SEVSELECT Procedure
EFFECT Statement
EFFECT name=effect-type (variables </ options>);
The EFFECT statement enables you to construct special collections of columns for design matrices. These collections are referred to as constructed effects to distinguish them from the usual model effects that are formed from continuous or classification variables, as discussed in the section GLM Parameterization of Classification Variables and Effects in Chapter 4, Shared Concepts.
You can specify the following effect-types:
- COLLECTION
specifies a collection effect that defines one or more variables as a single effect that has multiple degrees of freedom. The variables in a collection are considered as a unit for purposes of estimation and inference.
- MULTIMEMBER | MM
specifies a multimember classification effect whose levels are determined by one or more variables that appear in a CLASS statement.
- POLYNOMIAL | POLY
specifies a multivariate polynomial effect in the specified numeric variables.
- SPLINE
specifies a regression spline effect whose columns are univariate spline expansions of one or more variables. A spline expansion replaces the original variable with an expanded or larger set of new variables.
Table 3 summarizes the options available in the EFFECT statement.
Table 3: EFFECT Statement Options
| Option | Description |
|---|---|
| Collection Effects Options | |
| DETAILS | Displays the constituents of the collection effect |
| Multimember Effects Options | |
| DETAILS | Displays the levels of the multimember effect |
| NOEFFECT | Specifies that observations whose levels are all missing for the multimember variables should have 0 values in the corresponding design matrix columns |
| STDIZE | Standardizes the design matrix entries so that each observation has a sum of 1 |
| WEIGHT= | Specifies the weight variable for the contributions of each classification effect |
| Polynomial Effects Options | |
| DEGREE= | Specifies the degree of the polynomial |
| DETAILS | Displays details of the specified polynomial |
| MDEGREE= | Specifies the maximum degree of any variable in a term of the polynomial |
| NOSEPARATE | Treats the polynomial as a single effect with multiple degrees of freedom |
| STANDARDIZE= | Specifies centering and scaling suboptions for the variables that define the polynomial |
| Spline Effects Options | |
| BASIS= | Specifies the type of basis (B-spline basis or truncated power function basis) for the spline effect |
| DATABOUNDARY | Uses the extremes of the data as boundary knots for a B-spline basis |
| DEGREE= | Specifies the degree of the spline effect |
| DETAILS | Displays the knots and locations for each spline basis function |
| KNOTMAX= | Requests equally spaced right-side boundary knots starting at the variables’ maximum and ending at the KNOTMAX= value |
| KNOTMETHOD= | Specifies how to construct the knots for the spline effect |
| KNOTMIN= | Requests equally spaced left-side boundary knots starting at the KNOTMIN= value and ending at the variables’ minimum value |
| NATURALCUBIC | Specifies a natural cubic spline basis for the spline effect |
| SEPARATE | Treats the spline basis for each variable as a separate effect when multiple variables are specified |
| SPLIT | Treats each design matrix column as a separate effect for selection methods |
For more information about the syntax of these effect-types and how columns of constructed effects are computed, see the section EFFECT Statement in Chapter 4, Shared Concepts. Each effect-type has a DETAILS option that you can use to print tables that contain the details of the effect. However, these tables are produced only if you also specify PRINT=ALL option in the PROC SEVSELECT statement.