I think you want first.dose as you want the first occurence of dosedate within a dose, correct? But WHICH dose dates are missing? If you sorted your data by usubjid dose and dosedate then the first dosedate(s) would be the missing ones. I might look at Proc summary data=lab1 nway;      class usubjid dose;      where dosedate ne .;      var dosedate;      output lab min(dosedate) = firstdosedate; run; ... View more

Something like this? proc sql;       create table temp as        select a.usubjid, a.avisitn, count1/count2 as pct            (select usubjid, avisitn, count(usubjid) as count2 from dataset where trt1 ne '' group by avisitn ) a                left join            (select usubjid,avisitn,count(usubjid) as count1 from dataset where trt1 ne '' and drug='PLACEBO'  group by avisitn) b      where a.avisitn=b.avisitn; quit;     then you can merge with original with usubjid avisitn.... ... View more