Correction
if (POIDS>0 and substr(USAGE,2,2) NOT IN ('20','21')) then do;
Notes:
1. To filter a list that has more than one operand to evaluate, an equality operator is not appropriate . For an equality operator to work, you would have to work with just one operand
2. While you coded your IN the list correctly, all you need is NOT IN for all values that are not 20,21 to be true
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