At 03:43 PM 9/16/2009, Victor Mauricio Herrera wrote:
model 2 --> xi: logistic sex i.ageg if age>=30
i.ageg _Iageg_2-7 (naturally coded; _Iageg_2 omitted)
note: _Iageg_4 dropped because of collinearity
Logistic regression Number of obs = 8017
LR
chi2(4) = 2.35
Prob >
chi2 = 0.6713
Log likelihood = -5544.1939 Pseudo R2 = 0.0002
------------------------------------------------------------------------------
sex | Odds Ratio Std. Err. z P>|z| [95%
Conf. Interval]
-------------+----------------------------------------------------------------
_Iageg_3
| .9789833 .0734452 -0.28 0.777 .8451164 1.134055
_Iageg_5
| .9451487 .0748512 -0.71 0.476 .8092619 1.103853
_Iageg_6
| .9931979 .0670654 -0.10 0.919 .8700789 1.133739
_Iageg_7
| .8989579 .0765455 -1.25 0.211 .7607821 1.06223
------------------------------------------------------------------------------
Why is the age group 4 (40-49) dropped due to collinearity if there
are 610 males and 660 females in this stratum? More worrisome, why
is the age group 2 (20-29) still being used as reference when it
should have been dropped as a consequence of the "if" statement
(i.e. _Iage_3 should be the reference instead of _Iage_2)?
I believe group 4 is being used as the reference, not group 2. Group
2 is not in the analysis. Also, I replicate your model 2 in Stata
10.1...but when I run it in Stata 11 I get
. xi: logistic sex i.ageg if age>=30
i.ageg _Iageg_2-7 (naturally coded; _Iageg_2 omitted)
note: _Iageg_7 omitted because of collinearity
Logistic regression Number of obs = 8017
LR chi2(4) = 2.35
Prob > chi2 = 0.6713
Log likelihood = -5544.1939 Pseudo R2 = 0.0002
------------------------------------------------------------------------------
sex | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
_Iageg_3 | 1.08902 .0882688 1.05 0.293 .9290586 1.276523
_Iageg_4 | 1.112399 .0947198 1.25 0.211 .9414161 1.314437
_Iageg_5 | 1.051383 .0892948 0.59 0.555 .8901588 1.241807
_Iageg_6 | 1.104832 .0819438 1.34 0.179 .9553535 1.2777
_Iageg_7 | (omitted)
------------------------------------------------------------------------------
This labeling may be a little clearer -- you can see that group 7 is
the reference while group 2 is not in the analysis.
-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
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