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st: help with bys


From   Ziad El-Khatib <ziad.khatib@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: help with bys
Date   Thu, 28 Aug 2008 10:01:42 -0700

Hi!
I am dealing with cohort data for patients followed over different
visits, before starting treatment (coded -1) then on different time
points coded as 0, 2, 4, 8, 12, 20, 24, 36, 52, 78 and 104, also
number 999 will be used if it was an unexpected visit to clinic.

Not all patients came to all visits, so I am using -bys- command
(pasted below) to calculate total number of visits, excluding codes -1
and 999.
The command below is still calculating all visits by living . value if
visit=-1 or visit=999

bys studyid: gen nvisits = _N if visit>=0 &visit<999


Any tips?
Thank you and best regards
ziad

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