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Re: st: redundant dummies that are not dropped


From   Daniel Egan <dp.egan@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: redundant dummies that are not dropped
Date   Tue, 26 Oct 2004 09:45:09 -0400

Hi Thi, 

It sounds like you have two different sets of dummies - meaning that
commune dummies are not mutually exclusive of region dummies. One
dummy from each set of dummies should be dropped, but not a whole set.
The easiest way to visualize this is to add across columns, and note
if the summation is a constant - i.e. in the example below, Reg1 +
Com1 =[2,1,1,0] so they are not colinear.

e.g.
Ob     Reg1     Com1     Reg2       Com2
1       1            1          0             0
2       1            0          0             1
3       0            1          1             0
4       0            0          1             1 


Otherwise, there is a chance that there was an error in the coding of
the dummies.
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