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Re: st: Categorical variable in regression


From   Petya Alexandrova <p.alexandrova@montesquieu-instituut.nl>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Categorical variable in regression
Date   Wed, 23 Jan 2013 14:52:35 +0200 (EET)

I see. I am not sure if this works but you can try. Run the model 
without 'xi'. First create dummy variables out of your categorical 
variable.



tabulate region, gen(reg)

reg y reg*, noconst OR reg y reg1 reg2 reg3 reg4, noconst 



Btw, i saw the command with noomit somwehere written in the following 
way: xi, noomit: reg ... So, try typing your command like this:

xi, noomit: reg y i.region, noconst 



I hope at least one of the options works!



Petya











 >-------- Оригинално писмо --------

 >От: John Ataguba johnataguba@yahoo.co.uk

 >Относно: Re: st: Categorical variable in regression

 >До: "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>

 >Изпратено на: Сряда, 2013, Януари 23 14:00:39 EET



 
> Thanks Petua for your response.  Maybe my question was not clear enough.  Note that I have suppressed the constant term in my regression - xi: reg  y  i.region, noconst noomit -.  In that case there is no problem of the dummy trap.  Based on Stata's documentation, the regression model I have specified is expected to run.  Unfortunately, it is not running for me.  
 
> 
 
> I look forward to more response.
 
> 
 
> Regards
 
> 
 
> Jon
 
> 
 
> 
 
> >________________________________
 
> > From: Petya Alexandrova 
 
> >To: statalist@hsphsun2.harvard.edu 
 
> >Sent: Wednesday, 23 January 2013, 13:32
 
> >Subject: Re: st: Categorical variable in regression
 
> > 
 
> >In a regression with dummy variables one category is always omitted because it is the reference category. 
 
> >You can modify which category to be used as a reference group.
 
> >
 
> >Petya 
 
> >
 
> >
 
> >
 
> >
 
> >
 
> >>-------- Оригинално писмо --------
 
> >
 
> >>От: John Ataguba johnataguba@yahoo.co.uk
 
> >
 
> >>Относно: st: Categorical variable in regression
 
> >
 
> >>До: Statalist statalist mailing 
 
> >
 
> >>Изпратено на: Сряда, 2013, Януари 23 13:22:09 EET
 
> >
 
> >
 
> >
 
> >
 
> >> Dear Statalist members
 
> >
 
> >> I have a problem trying to let Stata display coefficients for all the categories of a categorical variable in a regression.
 
> >
 
> >> Consider the example below:
 
> >
 
> >> 
 
> >
 
> >> **********************************************
 
> >
 
> >> clear
 
> >
 
> >> set obs 200
 
> >
 
> >> set seed 12345
 
> >
 
> >> gen region = 1+int((4-1+1)*runiform())
 
> >
 
> >> gen y = 10+(56-10)*runiform()
 
> >
 
> >> xi: reg  y  i.region, noconst noomit
 
> >
 
> >> **********************************************
 
> >
 
> >> 
 
> >
 
> >> I expect Stata to display the coefficients for all the four categories of the categorical variable named "region".  However, Stata still omits one category.  Is there something I am doing wrong?
 
> >
 
> >> 
 
> >
 
> >> I am running Stata 12.1 (SE) on Windows 7 platform.  I also tried this in Stata 11.2 (IC) on Windows 7 platform and I obtained the same result.
 
> >
 
> >> 
 
> >
 
> >> I will appreciate your assistance.
 
> >
 
> >> 
 
> >
 
> >> Regards
 
> >
 
> >> 
 
> >
 
> >> Jon
 
> >
 
> >> 
 
> >
 
> >> *
 
> >
 
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> >
 
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> >
 
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> >
 
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> >
 
> >
 
> >   
 
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