# Re: st: Stratify analysis - logistic regression with dummies

 From Ricardo Ovaldia 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.
>
> Ricardo.
>
>
> Ricardo Ovaldia, MS
> Statistician
> Oklahoma City, OK
>
>
>
>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/

*
*   For searches and help try:
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*   http://www.ats.ucla.edu/stat/stata/
```