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st: Setting a reference value by rows for calculating differences


From   CHERYL BARNABE <ccbarnab@ucalgary.ca>
To   statalist@hsphsun2.harvard.edu
Subject   st: Setting a reference value by rows for calculating differences
Date   Fri, 17 Jun 2011 00:45:26 -0600

Hi there, trying to calculate the elapsed time from the baseline (or 1st visit) for each subject to subsequent follow-up visits, not the prior visit, but can't figure out the command to keep the first entry per subject as the reference, and need the visitmonth variable for reshaping purposes. Does anyone know how to do this? Thanks a million

study_id	visitdate	visitorder	visitmonth	visittime
01-1	30nov2006	1		0	
01-1	12feb2007	2					74
01-1	01may2007	3					78
01-1	14dec2007	4					227
01-1	30may2008	5					168
01-1	21nov2008	6					175
01-1	08may2009	7					168
01-10	21jun2007	1		0	
01-10	13sep2007	2					84
01-10	27mar2008	3					196
01-10	11sep2008	4					168
01-10	13jan2009	5					124
01-10	02apr2009	6					79

by study_id: gen visittime=(visitdate-visitdate[_n-1])
by study_id: generate visitmonth=(visitdate-visitdate[_n==1])
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