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RE: st: test for departure from linearity


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: test for departure from linearity
Date   Sun, 20 Nov 2005 15:59:18 -0000

On Raoul's original question, my answer is neither 
and both. A graphical "test", meaning examination, 
should complement a "statistical test", meaning 
hypothesis or significance test, and both should 
be useful. 

Nick 
n.j.cox@durham.ac.uk 

Mark Schaffer
 
> It sounds like you are interested in a RESET test.  This is sometimes,
> rather misleadingly, called an omitted variables test, but it's better
> thought of as a test for neglected nonlinearities in functional form.
> 
> Stata has a canned RESET test called -ovtest-.  -ivreset- 
> (downloadable
> from ssc-archives) will also work after -regress- and gives you more
> options.

> > What is the best way to test for a departure from linearity in a
> > regression model? Graphically or statistically? I ask this 
> because when I
> > use, for example graphical methods like: .lincheck or 
> .cprplot (component
> > plus residual plot), or .acprplot(augmented component plus 
> residual) the
> > variable that I test (age) looks very linear. However, when I do a
> > statistical test for it, it is not linear!
> > I first create a regression model with a dummy 
> variable(i.age) and then
> > compare it to a regression model with the same
> > variable but then as an ordinal variable (age). See below 
> for an example
> > (BP=blood pressure)
> >
> > xi: regress BP i.age
> > estimate store Model1
> > regress BP age
> > estimate store Model2
> > lrtest M2 M2
> >
> > My sample size is very large (approx 20,000 subjects), 
> could this be why
> > the test gives a significant result while the plots look 
> very linear?

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