The TTEST Procedure

An observation is omitted from the calculations if it has a missing value for either the CLASS variable, a CROSSOVER= variable, a PAIRED variable, the variable to be tested (in a one-sample or two-independent-sample design), or either of the two response variables (in a crossover design). If more than one variable or pair of variables is listed in the VAR statement, a missing value in one variable or pair does not eliminate the observation from the analysis of other nonmissing variables or variable pairs.