Re: st: RE: Multiple dummies - what does the constant measure?

[Jimmy Verner <jverner@earthlink.net>]

Thx. for the feedback.  Actually, the model would have some interval  
variables, too - I just oversimplified it.

What I'm really trying to do is fixed effects regression with more  
than one categorical variable.  I'm thinking it's not possible unless  
I interact the categorical variables, in which case my database would  
require 84 separate regressions (Category A (4) x Category B (21))  
slopes.

Jimmy Verner
Graduate Student
University of Texas at Dallas

On Dec 1, 2008, at 4:49 PM, Steichen, Thomas J. wrote:

> It is the mean in the A4*B4 category. Remember, you have categories  
> only, so speaking of this as an "intercept" is a little awkward. It  
> is, in the sense that it is the value when A1=0 and A2=0 and A3=0  
> and B1=0 and B2=0 and B3=0, but that is just the mean when A4=1 and  
> B4=1.
>
> Summarize Y when A4==1 and B4==1 and you'll see this is true:
>
>  summ Y if A4 & B4
>
> So, what do you mean when you say you want the intercept for B4?
> This: ?
>
>  summ Y if B4 (the average ignoring A)
>
> Should be the same as the _cons from a regression ignoring A:
>  regress Y B1 B2 B3
>
> Likewise for A4 (ignoring B)
>  summ Y if A4
> Or _cons from:
>  regress Y A1 A2 A3
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu 
> ] On Behalf Of Jimmy Verner
> Sent: Monday, December 01, 2008 5:21 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: Multiple dummies - what does the constant measure?
>
> Suppose you have two sets of categorical dummies:
>
> A1 A2 A3 A4
>
> B1 B2 B3 B4
>
> When you regress Y on them you must drop one category from each  
> dummy to avoid the dummy variable trap, i.e., Y = a + A1 + A2 + A3 +  
> B1 + B2 + B3 + e.
>
> So Stata's regression output looks like this:
>
> A1     0.826
> A2     0.224
> A3     0.187
> B1     0.376
> B2     0.997
> B3     0.736
> _cons  0.556
>
> Question:  What does the coefficient for the constant represent?  Is  
> it the intercept of A4 plus the intercept for B4?  If so, how does  
> one ascertain the intercepts for A4 and B4 separately? Does one pick  
> different dummies to omit and run the regression again?
>
> Thx.
>
> Jimmy Verner
> Graduate Student
> University of Texas at Dallas
>
> Jimmy L. Verner, Jr.
> Verner & Brumley, P.C.
> 3131 TurtleCreek Blvd.
> Penthouse Suite
> Dallas, Texas  75219
> 214.526.5234
> 214.526.0957.fax
> jverner@vernerbrumley.com
> www.vernerbrumley.com
>
> Board Certified, Family Law and Civil Trial Law,
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> Also admitted in New Mexico
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