Thank you for this response. I have come across more than one observation with the same ID/DOB/SEX triplet sort key. And you are correct. Comparing the Proc SORT vs. PROC SQL, the PROC SQL method doesn't have the same EQUALS behavior as PROC SORT, so when I attempted to pick the first record in the sort key based on PROC SQL or PROC SORT I have noticed slight differences in output. Yes you do get the same number of observations, but the distribution looks different.
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