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Re: st: "if" statement


From   Richard Williams <Richard.A.Williams.5@ND.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>, statalist <statalist@hsphsun2.harvard.edu>
Subject   Re: st: "if" statement
Date   Wed, 16 Sep 2009 17:01:09 -0500

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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