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 <[email protected]> wrote:
> From: Ricardo Ovaldia <[email protected]>
> Subject: st: Stratify analysis - logistic regression with dummies
> To: [email protected]
> 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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*
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