Previous Page | Next Page

The GENMOD Procedure

CLASS Variable Parameterization

Consider a model with one CLASS variable A with four levels, 1, 2, 5, and 7. Details of the possible choices for the PARAM= option follow.

EFFECT

Three columns are created to indicate group membership of the nonreference levels. For the reference level, all three dummy variables have a value of . For instance, if the reference level is 7 (REF=7), the design matrix columns for A are as follows:

Effect Coding

 

Design Matrix

A

A1

A2

A5

Parameter estimates of CLASS main effects that use the effect coding scheme estimate the difference in the effect of each nonreference level compared to the average effect over all four levels.

GLM

As in PROC GLM, four columns are created to indicate group membership. The design matrix columns for A are as follows:

GLM Coding

 

Design Matrix

A

A1

A2

A5

A7

Parameter estimates of CLASS main effects that use the GLM coding scheme estimate the difference in the effects of each level compared to the last level.

ORDINAL
THERMOMETER

Three columns are created to indicate group membership of the higher levels of the effect. For the first level of the effect (which for A is 1), all three dummy variables have a value of 0. The design matrix columns for A are as follows:

Ordinal Coding

 

Design Matrix

A

A2

A5

A7

The first level of the effect is a control or baseline level. Parameter estimates of CLASS main effects that use the ORDINAL coding scheme estimate the effect on the response as the ordinal factor is set to each succeeding level. When the parameters for an ordinal main effect have the same sign, the response effect is monotonic across the levels.

POLYNOMIAL
POLY

Three columns are created. The first represents the linear term (), the second represents the quadratic term (), and the third represents the cubic term (), where is the level value. If the CLASS levels are not numeric, they are translated into 1, 2, 3, according to their sorting order. The design matrix columns for A are as follows:

Polynomial Coding

 

Design Matrix

A

APOLY1

APOLY2

APOLY3

REFERENCE
REF

Three columns are created to indicate group membership of the nonreference levels. For the reference level, all three dummy variables have a value of 0. For instance, if the reference level is 7 (REF=7), the design matrix columns for A are as follows:

Reference Coding

 

Design Matrix

A

A1

A2

A5

Parameter estimates of CLASS main effects that use the reference coding scheme estimate the difference in the effect of each nonreference level compared to the effect of the reference level.

ORTHEFFECT

The columns are obtained by applying the Gram-Schmidt orthogonalization to the columns for PARAM=EFFECT. The design matrix columns for A are as follows:

Orthogonal Effect Coding

 

Design Matrix

A

AOEFF1

AOEFF2

AOEFF3

ORTHORDINAL
ORTHOTHERM

The columns are obtained by applying the Gram-Schmidt orthogonalization to the columns for PARAM=ORDINAL. The design matrix columns for A are as follows:

Orthogonal Ordinal Coding

 

Design Matrix

A

AOORD1

AOORD2

AOORD3

ORTHPOLY

The columns are obtained by applying the Gram-Schmidt orthogonalization to the columns for PARAM=POLY. The design matrix columns for A are as follows:

Orthogonal Polynomial Coding

 

Design Matrix

A

AOPOLY1

AOPOLY2

AOPOLY5

ORTHREF

The columns are obtained by applying the Gram-Schmidt orthogonalization to the columns for PARAM=REFERENCE. The design matrix columns for A are as follows:

Orthogonal Reference Coding

 

Design Matrix

A

AOREF1

AOREF2

AOREF3

Previous Page | Next Page | Top of Page