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st: RE: dummy dropped in xtgee

From   "Maarten Buis" <>
To   <>
Subject   st: RE: dummy dropped in xtgee
Date   Fri, 20 Apr 2007 17:38:30 +0200

--- Jessica Kiefte-de Jong wrote:
> I am using GEE in STATA for analysis of an interventionstudy where data
> collection was performed every 6 months and I am assessing time as
> categorial variable by using dummies. However, STATA dropped the dummy I
> used as reference catagory (Dtime6). How can I fix this problem?

There is no problem to solve. Stata does exactly what it should be doing. 
An observation can fall into one of four categories: 6, 12, 18, and 24 
months after intervention. The dummies tell you whether or not a case is 
in category 6, 12, 18 or 24. If you know that a case is not in categories 
12, 18 and 24 (i.e. Dtime12 Dtime 18 Dtime24 are all 0) than we know that 
it must be in category 6. If we know the case is in one of the other 
categories (i.e. one of Dtime12, Dtime 18, Dtime24 is 1) we know that that 
case is not in category 6.  As a result, the dummy Dtime6 contains no 
information that isn't present in the other dummies and must be removed. 

The remaining dummies will measure how different a case is relative to 
period 6. 

Hope this helps,

Maarten L. Buis
Department of Social Research Methodology 
Vrije Universiteit Amsterdam 
Boelelaan 1081 
1081 HV Amsterdam 
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434 

+31 20 5986715


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