A CLASS statement giving the name of the classification (or grouping) variable must accompany the PROC TTEST statement in the two-independent-sample case. It should be omitted for the one-sample, paired, and AB/BA crossover designs. If it is used without the VAR statement, all numeric variables in the input data set (except those appearing in the CLASS, BY, FREQ, or WEIGHT statement) are included in the analysis.

The classification variable must have two, and only two, levels. PROC TTEST divides the observations into the two groups for the test by using the levels of this variable. You can use either a numeric or a character variable in the CLASS statement.

Class levels are determined from the formatted values of the CLASS variable. Thus, you can use formats to define group levels. See the discussions of the FORMAT procedure, the FORMAT statement, formats, and informats in SAS Language Reference: Dictionary.