Dear Co-Stata-Users,
I encounter a behavior I don't understand. I'm running a regression
(stripped down to its bones in order to better present my problem) with xi
and an if expression:
. xi: reg qa_ek i.r2 if r2>15
i.r2 _Ir2_15-93 (naturally coded; _Ir2_15 omitted)
Source | SS df MS Number of obs =
85825
-------------+------------------------------ F( 2, 85822) =
132.51
Model | 207587.248 2 103793.624 Prob > F =
0.0000
Residual | 67223628.8 85822 783.291333 R-squared =
0.0031
-------------+------------------------------ Adj R-squared =
0.0031
Total | 67431216.0 85824 785.691834 Root MSE =
27.987
----------------------------------------------------------------------------
--
qa_ek | Coef. Std. Err. t P>|t| [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
_Ir2_91 | -4.046854 .2492706 -16.23 0.000 -4.535422
-3.558285
_Ir2_92 | (dropped)
_Ir2_93 | -2.513805 .2658109 -9.46 0.000 -3.034792
-1.992818
_cons | 22.55939 .2058561 109.59 0.000 22.15592
22.96287
----------------------------------------------------------------------------
--
My questions are,
- why does STATA use r2==15 as the omitted category when I excluded this
category in the if exp? Is an if exp useless with xi when the same var is in
the if exp and handled by xi?
- why is r2=92 dropped? Is it because the omitted category was chosen
wrongly (there should be no with the data)?
Maybe somebody could possibly point me to an explanation and a remedy?
TIA
Sincerely
Dankwart Plattner
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