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Re: st: combine indicator values


From   Nikolaos Pandis <npandis@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: combine indicator values
Date   Mon, 23 Nov 2009 07:29:32 -0800 (PST)

Dear Maarten, 
This is excatly what I needed.
Many thanks to all the people that responed to my question.
Best wishes,
Nick

--- On Mon, 11/23/09, Maarten buis <maartenbuis@yahoo.co.uk> wrote:

> From: Maarten buis <maartenbuis@yahoo.co.uk>
> Subject: Re: st: combine indicator values
> To: statalist@hsphsun2.harvard.edu
> Date: Monday, November 23, 2009, 1:07 PM
> --- Nikolaos Pandis wrote:
> > If I only enter child2 and child3 (independent
> variables) 
> > in the model how would child1 be used as a reference
> for 
> > child2 and child3?
> 
> Lets consider the simple case of a linear model:
> 
> y = b0 + b1 child2 + b2 child3
> 
> Now look at what happens at the three possible
> scenarios:
> 
> 1 kid means child2 = 0 and child3 = 0:
> y = b0 + b1*0 + b2*0 = b0
> 
> 2 kids means child2 = 1 and child3 = 0:
> y = b0 + b1*1 + b2*0 = b0 + b1
> 
> 3+ kisd means child2 = 0 and child3 = 1:
> y = b0 + b1*0 + b2*1 = b0 + b2
> 
> So, b0 measure the expected value of y when
> there is 1 kid, b1 and b2 measures the 
> difference in y relative to the situation 
> with only 1 kid. 
> 
> So leaving child1 out makes it the reference
> category.
> 
> In the example below you can see that the
> same logic also works in a logistic regression.
> 
> *------------------- begin example ------------------
> sysuse nlsw88, clear
> 
> // create a categorical variable from dummy variables
> gen marst = cond(never_married, 1,     
>         ///
>             cond(married, 2,
> 3))               
> ///
>             if
> !missing(married, never_married)
> label define marst 1 "never married"     
>       ///
>                
>    2 "married"       
>           ///
>                
>    3 "divorced/widowed"
> 
> //set the reference category to be divorced/widowed
>  char marst[omit] 3
>  
> // both -xi- and using the dummy variables lead to
> // exactly the same results
>  xi : logit union i.marst
>  logit union never_married married
> *----------------- end example ---------------------
> ( For more on how to use examples I sent to statalist see:
> http://www.maartenbuis.nl/stata/exampleFAQ.html )
> 
> Hope this helps,
> Maarten
> 
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
> 
> http://www.maartenbuis.nl
> --------------------------
> 
> 
>       
> 
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