PROC PHREG: CLASS Statement :: SAS/STAT(R) 9.2 User's Guide, Second Edition

The PHREG Procedure

CLASS Statement

CLASS variable <(options)> <variable <(options)> ...> </ options> ;

The CLASS statement names the categorical variables to be used in the analysis. The CLASS statement must precede the MODEL statement. You can specify various options for each variable by enclosing them in parentheses after the variable name. You can also specify global options for the CLASS statement by placing them after a slash (/). Global options are applied to all the variables specified in the CLASS statement. If you specify more than one CLASS statement, the global options specified in any one CLASS statement apply to all CLASS statements. However, individual CLASS variable options override the global options.

CPREFIX= n

specifies that, at most, the first n characters of a CLASS variable name be used in creating names for the corresponding dummy variables. The default is $\text{[math]}$ , where $\text{[math]}$ is the formatted length of the CLASS variable.

DESCENDING

DESC

reverses the sorting order of the categorical variable.

LPREFIX= n

specifies that, at most, the first n characters of a CLASS variable label be used in creating labels for the corresponding dummy variables.

MISSING

allows a missing value (for example,‘.’ for a numeric variable and blanks for a character variable) as a valid value for the CLASS variable.

ORDER=DATA | FORMATTED | FREQ | INTERNAL

specifies the sorting order for the categories of cateogrical variables. This ordering determines which parameters in the model correspond to each level in the data, so the ORDER= option can be useful when you use the CONTRAST statement. When the default ORDER=FORMATTED is in effect for numeric variables for which you have supplied no explicit format, the levels are ordered by their internal values. The following table shows how PROC PHREG interprets values of the ORDER= option.

Value of ORDER=	Levels Sorted By
DATA	order of appearance in the input data set
FORMATTED	external formatted value, except for numeric variables with no explicit format, which are sorted by their unformatted (internal) value
FREQ	descending frequency count; levels with the most observations come first in the order
INTERNAL	unformatted value

By default, ORDER=FORMATTED. For FORMATTED and INTERNAL, the sort order is machine dependent. For more information about sorting order, see the chapter on the SORT procedure in the Base SAS Procedures Guide and the discussion of BY-group processing in SAS Language Reference: Concepts.

PARAM=keyword

specifies the parameterization method for the categorical variable or variables. Design matrix columns are created from CLASS variables according to the following coding schemes. The default is PARAM=REF. If PARAM=ORTHPOLY or PARAM=POLY, and the CLASS levels are numeric, then the ORDER= option in the CLASS statement is ignored, and the internal, unformatted values are used. See the section CLASS Variable Parameterization for further details.

EFFECT: specifies effect coding.
GLM: specifies less-than-full-rank, reference-cell coding; this option can be used only as a global option.
ORDINAL: specifies the cumulative parameterization for an ordinal CLASS variable.
POLYNOMIAL | POLY: specifies polynomial coding.
REFERENCE | REF: specifies reference cell coding.
ORTHEFFECT: orthogonalizes PARAM=EFFECT
ORTHORDINAL: orthogonalizes PARAM=ORDINAL.
ORTHPOLY: orthogonalizes PARAM=POLYNOMIAL.
ORTHREF: orthogonalizes PARAM=REFERENCE.

The EFFECT, POLYNOMIAL, REFERENCE, and ORDINAL coding schemes and their orthogonal parameterizations are full-rank parameterization. The REF= option in the CLASS statement determines the reference level for the EFFECT, REFERENCE, and their orthogonal parameterizations.

Parameter names for a CLASS predictor variable are constructed by concatenating the CLASS variable name with the CLASS levels. However, for the POLYNOMIAL and orthogonal parameterizations, parameter names are formed by concatenating the CLASS variable name and keywords that reflect the parameterization.

REF=’level’ | keyword

specifies the reference level for PARAM=EFFECT or PARAM=REF. For an individual variable, you can specify a specific level of the variable in the REF= option. For a global or individual variable REF= option, you can use one of the following keywords. The default is REF=LAST.

FIRST: designates the first ordered level as reference.
LAST: designates the last ordered level as reference.

TRUNCATE

specifies that class levels should be determined using no more than the first 16 characters of the formatted values of CLASS variables. This is a global option, not an individual CLASS variable option.

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