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Re: st: centering explanatory variables around zero


From   Nikolaos Pandis <npandis@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: centering explanatory variables around zero
Date   Sun, 9 Aug 2009 11:47:58 -0700 (PDT)

Dear Richard 

Thank you for the clear reply and the reference.

I did use the xi command for the categorical variable-I apologise for not claryfying it.

best wishes,

Nick




--- On Sun, 8/9/09, Richard Williams <Richard.A.Williams.5@ND.edu> wrote:

> From: Richard Williams <Richard.A.Williams.5@ND.edu>
> Subject: Re: st: centering explanatory variables around zero
> To: "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>, "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
> Date: Sunday, August 9, 2009, 10:31 PM
> At 12:50 PM 8/9/2009, Nikolaos Pandis
> wrote:
> >I have the following model
> >
> >y=a + b1x1 + b2x2
> >
> >y and x2 are continuous
> >x1=categorical with 3 levels
> 
> You shouldn't include x1 as is.  You should break it
> up into dummy 
> variables.  e.g. if for x1, 1 = Catholic, 2 =
> Protestant, 3 = Others, 
> it makes no sense to treat it as continous.
> 
> >I find a large difference in the p-values for the
> explanatory 
> >variables between the interaction and no-interaction
> model.
> >
> >I centered for the continuous variable x2 ((sum x2, gen
> cx2=x2-r(mean)).
> >
> >I get similar p values for interaction and
> no-interaction models.
> >
> >Question: How about the categorical explanatory
> variable? Is it 
> >appropriate to center this variable also? Does it make
> any sense?
> 
> No.  Again, it shouldn't be in there in the first
> place; and if you 
> did dichotomize it you still wouldn't center.
> 
> This handout goes over the advantages of centering and how
> to 
> interpret results:
> 
> http://www.nd.edu/~rwilliam/stats2/l53.pdf
> 
> 
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> HOME:   (574)289-5227
> EMAIL:  Richard.A.Williams.5@ND.Edu
> WWW:    http://www.nd.edu/~rwilliam
> 
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