In reply to JON SELTZER.

As the example in this article shows, The output from PROC TTEST includes a table that has rows for Group1, Group2, and DIF(Group1-Group2). The first row specifies the value that is used for Group1.

The easiest way to control which group is Group1 and which is Group2 is to use the ORDER=DATA option on the PROC TTEST statement, as I did in these examples. Then Group1 is the first category that appears in the input data set and Group2 is the second group in the data. If you don't specify ORDER=DATA, then the sort-order is used. The sort order depends on your locale/language, so the easiest way to determine sort order is to run PROC SORT on your system:

data a;
x="_1"; output; x="W"; output; x="w"; output;
run;
proc sort data=a; by x;run;
proc print; run;