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st: RE: regression assumption question

From   "Lachenbruch, Peter" <>
To   <>
Subject   st: RE: regression assumption question
Date   Mon, 9 Mar 2009 12:58:02 -0700

My preference is to use the preop measure as a covariate.  If you use
the change, you are essentially forcing the preop to have a coefficient
of 1.  Sometimes people use the preop as a covariate for the change
score - this automatically induces a fairly high correlation with preop
- if you're not careful, you can believe it.


Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001

-----Original Message-----
[] On Behalf Of moleps islon
Sent: Monday, March 09, 2009 11:49 AM
Subject: st: regression assumption question

Dear listers,
I've got data on 300 patients preop and postop using the VAS scale
(ordinal scale). I'm trying to locate factors predicting improvement
postop. However there are several questions pertaining to this that
I'm unsure of. 1)Do I violate the assumption of independence? I assume
there would be some correlation between preop and postop within the
patients. 2)Would you recommend using delta (preop-postop) as the
dependent variable or postop alone? The analyses so far show some
heteroscedasticity-in case i violate the independence assumption- is
it possible to do add both vce(robust) and vce(cluster id) ?

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