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st: Is this the right code if I want to compare group 1 vs group 4 in a logistic regression model?


From   "Meems, LMG" <l.m.g.meems@umcg.nl>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: Is this the right code if I want to compare group 1 vs group 4 in a logistic regression model?
Date   Wed, 4 Dec 2013 14:15:48 +0000

Hello Statalisters,

After a couple of days filled with STATA and database work, I really need a check if what I'm doing is right..

At the moment I'm looking at the predicted effect from a continuous variable (Y) on a couple of other parameters.
I decided to split the continuous variable in 4 groups: thereby following it's clinical reference values (e.g. sufficient, insufficient etc.).

After this step I wanted to fit this variable in a regression model, using logistic regression (as I thought that dividing it in groups turned the continuous variable into a categorical one..). So far, so good..

However, let's say I now want to compare the lowest group (0) with the highest group (3) and the effects on age and sex.
The code I used to do this is:
Char (Y) [omit] 3
Xi: logit i.Y + age sex

This resulted in coefficients for age and sex, but also resulted in 2 ommitted values, namely group 1 and 2. With the comment that group 1 and 2 !=0 and predicted failure perfectly.

So, this result made me doubting about the code. Is this the right code to use and what exactely do these 2 ommitted values mean? Is it a result from the code I made (that would be the good scenario) or is it something wrong and should I correct for it (or even correct the code)?

Looking forward to the answer and thank you!!

Laura

=================================

Laura M.G. Meems, MD/PhD student
University Medical Center Groningen (UMCG)
Dept. of Experimental Cardiology, AB43
P.O. Box 30.001
9700 RB Groningen, The Netherlands
P +31 50 361 5054 | F +31 50 361 1347 |

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