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Re: st: Factor variable notation vs. hand made dummy vars


From   brendan.halpin@ul.ie (Brendan Halpin)
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Factor variable notation vs. hand made dummy vars
Date   Mon, 06 Feb 2012 15:41:52 +0000

To put the "why" back one step, the immediate reason is evident from the
output

| . logit for mpg d2-d5
| 
| note: d2 != 0 predicts failure perfectly
|       d2 dropped and 8 obs not used
| 
| [...]
| 
| . logit for mpg ib1.rep78
| 
| note: 1.rep78 != 0 predicts failure perfectly
|       1.rep78 dropped and 2 obs not used
| 
| note: 2.rep78 != 0 predicts failure perfectly
|       2.rep78 dropped and 8 obs not used
| 
| note: 5.rep78 omitted because of collinearity
| 
| [...]

You end up fitting different models on different data. 

The question is now why do the formulations behave differently, and
which is the better default?

Brendan

-- 
Brendan Halpin,   Department of Sociology,   University of Limerick,   Ireland
Tel: w +353-61-213147  f +353-61-202569  h +353-61-338562;  Room F1-009 x 3147
mailto:brendan.halpin@ul.ie    ULSociology on Facebook: http://on.fb.me/fjIK9t
http://teaching.sociology.ul.ie/bhalpin/wordpress         twitter:@ULSociology
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