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Re: st: AW: "if" statement


From   Victor Mauricio Herrera <vherrera@wisc.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: AW: "if" statement
Date   Wed, 16 Sep 2009 20:14:57 -0500

Thanks to all for your advise.
Victor.


----- Original Message -----
From: Martin Weiss <martin.weiss1@gmx.de>
Date: Wednesday, September 16, 2009 4:04 pm
Subject: st: AW: "if" statement
To: statalist@hsphsun2.harvard.edu


>  <>
>  
>  http://www.stata.com/statalist/archive/2004-07/msg00763.html
>  
>  
>  HTH
>  Martin
>  
>  -----Ursprüngliche Nachricht-----
>  Von: owner-statalist@hsphsun2.harvard.edu
>  [mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Victor Mauricio
>  Herrera
>  Gesendet: Mittwoch, 16. September 2009 22:44
>  An: statalist
>  Cc: Leonelo Bautista
>  Betreff: st: "if" statement
>  
>  I'd appreciate if somebody could explain the following behavior of 
> the "if"
>  statement when used with "logistic" (I'm running STATA IC/10.1). 
>  
>  webuse nhanes2f
>  gen ageg=2 if age>=20 & age<30
>  replace ageg=3 if age>=30 & age<40
>  replace ageg=4 if age>=40 & age<50
>  replace ageg=5 if age>=50 & age<60
>  replace ageg=6 if age>=60 & age<70
>  replace ageg=7 if age>=70
>  replace sex=0 if sex==2
>  
>  model 1 --> xi: logistic sex i.ageg
>  i.ageg            _Iageg_2-7          (naturally coded; _Iageg_2 omitted)
>  
>  Logistic regression                               Number of obs   =
>  10337
>                                                    LR chi2(5)      =
>  2.80
>                                                    Prob > chi2     =
>  0.7302
>  Log likelihood =  -7150.626                       Pseudo R2       =
>  0.0002
>  
>  ----------------------------------------------------------------------------
>  --
>           sex | Odds Ratio   Std. Err.      z    P>|z|     [95% Conf.
>  Interval]
>  -------------+--------------------------------------------------------------
>  --
>      _Iageg_3 |   .9761655   .0632659    -0.37   0.710     .8597191
>  1.108384
>      _Iageg_4 |   .9971218   .0696639    -0.04   0.967     .8695188
>  1.143451
>      _Iageg_5 |   .9424283    .065592    -0.85   0.394     .8222534
>  1.080167
>      _Iageg_6 |   .9903392   .0554211    -0.17   0.862     .8874609
>  1.105144
>      _Iageg_7 |   .8963705   .0683978    -1.43   0.152     .7718562
>  1.040971
>  ----------------------------------------------------------------------------
>  --
>  
>  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)?
>  
>  model 3 --> xi: logistic sex i.ageg if age<70
>  i.ageg            _Iageg_2-7          (naturally coded; _Iageg_2 omitted)
>  
>  note: _Iageg_7 dropped because of collinearity
>  
>  Logistic regression                               Number of obs   =
>  9352
>                                                    LR chi2(4)      =
>  0.86
>                                                    Prob > chi2     =
>  0.9303
>  Log likelihood = -6472.0855                       Pseudo R2       =
>  0.0001
>  
>  ----------------------------------------------------------------------------
>  --
>           sex | Odds Ratio   Std. Err.      z    P>|z|     [95% Conf.
>  Interval]
>  -------------+--------------------------------------------------------------
>  --
>      _Iageg_3 |   .9761655   .0632659    -0.37   0.710     .8597191
>  1.108384
>      _Iageg_4 |   .9971218   .0696639    -0.04   0.967     .8695188
>  1.143451
>      _Iageg_5 |   .9424283    .065592    -0.85   0.394     .8222534
>  1.080167
>      _Iageg_6 |   .9903392   .0554211    -0.17   0.862     .8874609
>  1.105144
>  ----------------------------------------------------------------------------
>  --
>  
>  Now the "if" statement seems to work fine, as subjects with age>=70 are
>  excluded (i.e. the _Iage_7 group has been dropped!)
>  
>  This also occurs if I run these models using STATA IC/9.2 or if one models
>  another dichotomous variable using a different dataset.
>  
>  Many thanks,
>  
>  
>  VICTOR M. HERRERA MD. MS.
>  Research Assistant
>  Population Health Sciences Department
>  University of Wisconsin
>  610 Walnut St. 626 WARF
>  Madison, WI 53726
>  (608) 265-3686
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