The INVALUE statement creates the numeric informat EVAL., which converts the letters to their numeric equivalents. The letters O (Outstanding), S (Superior), E (Excellent), C (Commendable), and N (None) correspond to the numbers 4, 3, 2, 1, and 0, respectively. 
   invalue eval 'O'=4
                'S'=3
                'E'=2
                'C'=1
                'N'=0;
run;