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Re: st: Categorical variable in regression


From   John Ataguba <johnataguba@yahoo.co.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Categorical variable in regression
Date   Wed, 23 Jan 2013 13:37:13 +0000 (GMT)

Dear Petya

Thanks very much for the second option :).  I already know how to use the first option but wanted a quicker method.

Regards

Jon


>________________________________
> From: Petya Alexandrova <p.alexandrova@montesquieu-instituut.nl>
>To: statalist@hsphsun2.harvard.edu 
>Sent: Wednesday, 23 January 2013, 14:52
>Subject: Re: st: Categorical variable in regression
> 
>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
>
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>
>
>
>>-------- 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
>
>> >
>
>> >
>
>> >
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>> >
>
>> >> 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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