The CLASS statement names the classification variables to be used in the model. Typical classification variables are TREATMENT, SEX, RACE, GROUP, and REPLICATION. If you specify the CLASS statement, it must appear before the MODEL statement.

By default, class levels are determined from the entire formatted values of the CLASS variables. Note that this represents a slight change from previous releases in the way in which class levels are determined. Before SAS 9, class levels were determined using no more than the first 16 characters of the formatted values. If you want to revert to this previous behavior, you can use the TRUNCATE option in the CLASS statement. In any case, you can use formats to group values into levels.

See the discussion of the FORMAT procedure in the Base SAS Procedures Guide, and the discussions of the FORMAT statement and SAS formats in SAS Language Reference: Dictionary.

The GLM procedure displays a table summarizing the CLASS variables and their levels, and you can use this to check the ordering of levels and, hence, of the corresponding parameters for main effects. If you need to check the ordering of parameters for interaction effects, use the E option in the MODEL, CONTRAST, ESTIMATE, and LSMEANS statements. See the section Parameterization of PROC GLM Models for more information.

You can specify the following option in the CLASS statement after a slash (/):