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Re: st: Stratify analysis - logistic regression with dummies


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Stratify analysis - logistic regression with dummies
Date   Thu, 5 Jun 2008 09:57:09 -0700 (PDT)

Dear _all,

I did not get a reply to my previous post. So either is too simple and not worth answering or it was not clear. So let me ask it this way:

if  
. xi:logistic low i.race age

and

. xi:logistic low i.race age if race==1 | race==2

do not produce the same odds ratio for _Irace_2, does that indicate an interaction between race and age? and, if I am interested in reporting the odds ratio for _Irace_2, which model's result should be used?

Thank you,
Ricardo 

Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


--- On Wed, 6/4/08, Ricardo Ovaldia <ovaldia@yahoo.com> wrote:

> From: Ricardo Ovaldia <ovaldia@yahoo.com>
> Subject: st: Stratify analysis - logistic regression with dummies
> To: statalist@hsphsun2.harvard.edu
> Date: Wednesday, June 4, 2008, 9:14 AM
> I am confused by some of the result that I got. I will
> illustrate using Hosmer & Lemeshow' low weight
> data:
> 
> . use http://www.stata-press.com/data/r10/lbw.dta
> (Hosmer & Lemeshow data)
> 
> if I fit
> 
> . xi:logistic low i.race
> 
> and then fit
> 
> . xi:logistic low i.race if race==1 | race==2
> 
> and
> 
> . xi:logistic low i.race if race==1 | race==3
> 
> I get the same OR for  _Irace_2  and _Irace_3 as I do for
> the full model. This is as expected because the dummies are
> ortogonal to each other.
> 
> However, when a covariate is added to the model, the same
> is not true anymore:
> 
>  
> . xi:logistic low i.race age
> 
>          low | Odds Ratio   Std. Err.      z    P>|z| 
> [95% Conf. Interval]
> -------------+-------------------------------------------------------------
>     _Irace_2 |   2.106974   .9932407     1.58   0.114  
> .8363679  5.307878
>     _Irace_3 |   1.767748   .6229325     1.62   0.106  
> .8860686  3.526738
>          age |   .9612592   .0311206    -1.22   0.222  
> .9021588  1.024231
> ---------------------------------------------------------------------------
> 
> . xi:logistic low i.race age if race==1 | race==2
> 
> ---------------------------------------------------------------------------
>          low | Odds Ratio   Std. Err.      z    P>|z| 
> [95% Conf. Interval]
> -------------+-------------------------------------------------------------
>     _Irace_2 |   2.155207   1.021287     1.62   0.105   
> .8513944  5.45566
>          age |   .9705512   .0376446    -0.77   0.441   
> .8995039  1.04721
> ---------------------------------------------------------------------------
> 
> . xi:logistic low i.race age if race==1 | race==3
> 
> ---------------------------------------------------------------------------
>          low | Odds Ratio   Std. Err.      z    P>|z| 
> [95% Conf. Interval]
> -------------+-------------------------------------------------------------
>     _Irace_3 |   1.724551   .6098827     1.54   0.123   
> .8622856  3.449063
>          age |   .9440875   .0340586    -1.59   0.111   
> .8796392  1.013258
> ---------------------------------------------------------------------------
> 
> 
> There is no missing data.
> 
> 
> I am very confused about which OR to reports and what are
> the differences in these models. I was not expecting these
> results.
> 
> Thank you in advance,
> Ricardo.
> 
> 
> Ricardo Ovaldia, MS
> Statistician 
> Oklahoma City, OK
> 
> 
> 
> 
>       
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