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


From   "Steichen, Thomas J." <SteichT@RJRT.com>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Multiple dummies - what does the constant measure?
Date   Mon, 1 Dec 2008 17:49:16 -0500

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,
Texas Board of Legal Specialization
Also admitted in New Mexico
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