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Re: st: Including constant?
From
Richard Williams <[email protected]>
To
[email protected], [email protected]
Subject
Re: st: Including constant?
Date
Wed, 01 Jun 2011 10:41:54 -0500
At 07:23 AM 6/1/2011, lreine ycenna wrote:
Do we typically not have to include constants in regression tables
when presenting them?
You can probably say most or all of what you want to say without
including the constant. After all, how many times do you see a paper
discussing the values of the constants?
But, I personally always include them. It is particularly useful if
somebody wants to calculate a yhat value for some combination of
values for the Xs. How useful the constant is as a standalone value
depends on the coding of the Xs. The constant is the value someone
would have if they had a value of 0 for every X. Often such a person
cannot exist because one or more variables cannot take on a value of
0, e.g. nobody has 0 height, and nobody gets a score of 0 on a test
scaled to range between 400 and 1600. However, if you center the Xs
(subtract the mean from each case for each X) then the constant
becomes the predicted score for a person who has average values on
every X. Such a person (or someone close to it) may actually exist,
making the constant more interpretable as a stand-alone number, i.e.
the constant is the score a totally average person would be expected
to have. Or, if the model just has dummy variables (female, black)
then the constant could be the average score for, say, a male white.
-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME: (574)289-5227
EMAIL: [email protected]
WWW: http://www.nd.edu/~rwilliam
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