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st: Subtracting in Variable by a Group


From   Arya Eskamani <Arya.Eskamani@assist-rx.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Subtracting in Variable by a Group
Date   Fri, 29 Mar 2013 16:10:01 +0000

Hello and thank you in advance

I'm attempting to find the difference in days between events within different groups.  The date is formatted %td (daily). Here's an example of my dataset:

group	event	date
A	1	08mar2013
A	2	10mar2013
A	3	15mar2013
B	1	10sep2012
B	2	15sep2012
C	1	07aug2011
C	2	23aug2011
C	3	23aug2011
C	4	24aug2011
D	1	25dec2012
E	1	01jan2013

I want to generate a new variable, say "time" , that calculates the difference in days from one event occurring to the next event in between the groups.  For example, in group A there are 2 days between even 1 and event 2 and then 5 days from event 2 to event 3, and similarly for others.  The example below shows what I'm trying to achieve:

group	event	date	time
A	1	08mar2013	.
A	2	10mar2013	2
A	3	15mar2013	5
B	1	10sep2012	.
B	2	15sep2012	5
C	1	07aug2011	.
C	2	23aug2011	16
C	3	23aug2011	0
C	4	24aug2011	1
D	1	25dec2012	.
E	1	01jan2013	.

Notice that the time variable is blank for 1st events because there is no previous event to subtract from.  I've never ran an operation within a column before and have been doing this in excel but would like to just write a do file on this.  Any help is much appreciated.  Thank you again.
 

Arya Eskamani


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