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st: Calculating a maximum out of a subset in a sequence

From   <>
Subject   st: Calculating a maximum out of a subset in a sequence
Date   Mon, 3 Oct 2011 18:16:09 -0400

Dear list:

I have a panel dataset with a subject identifier (patid), a time
identifier (week1), and two variables of interest (C4NAIVC and
CD4MEMC), some of which contain missing observations for some
subjects at some time points. I am interested in obtaining, for each
subject, the latest timepoint when either C4NAIVC and CD4MEMC or both
were available.

I tried:

. bysort patid: egen LatestNaive = max(WEEK1) if (C4NAIVC ~=.|CD4MEMC ~=.)
(252 missing values generated)

But this simply produced a missing value in the record that contained
a missing C4NAIVC or CD4MEMC. It still took all week1 values into
account to calculate the maximum. So, if somebody had a missing value
at week 96 but not at week 196, LatestNaive was still 196.

Any suggestions?
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