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st: First visit to clinic

From   "Paul O'Brien" <>
To   statalist <>
Subject   st: First visit to clinic
Date   Mon, 20 Sep 2010 14:24:41 +0100

i want to create a new variable with the date of the first visit of each patient:

id		date		date_of_first_visit
N-00-3558	07/04/2008	07/04/2008
N-00-3558	09/04/2008	07/04/2008
N-00-3558	14/05/2008	07/04/2008
N-00-3563	05/06/2007	
N-00-3563	31/03/2010	
N-00-3565	05/06/2007	
N-00-3565	13/06/2008	
N-00-3565	13/06/2008	
N-00-3565	05/05/2009	
N-00-3565	04/03/2010	
N-00-3583	09/09/2008	
N-00-3583	27/05/2009	

i know this should be easy by i am having trouble using n_1

i have tried:
.sort date
.by id, sort: gen visit=_n==1

but the latter does not have the date in order, only id.

any help gratefully received.


Dr Paul O'Brien
Raymede Clinic
Westside Contraceptive Services
Central London Community Healthcare
Exmoor St
W10 6DZ
+44 (0)20 8962 4450

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