Thanks to Al Best for his suggestion. However, I cannot find any
information about the intercept. I understand about centering
continuous predictors in a polynomial context. The regression
coefficients all make sense. What doesn't make clear sense is the
value of the Y intercept.
For example, if I use a sample data set (Facsalarydummyexamp.jmp)
(Download:
http://www.psych.northwestern.edu/psych/people/faculty/bailey/Facsalarydummyexamp.jmp)
if I regress Salary on Merit, Years, and their interaction, using
the cross button in the "fit model" box, I get 55.305 for the
intercept. If, however, I do this by first creating deviation scores
by hand and then creating a product of deviation variables for the
interaction term, I get exactly the same value for the regression
coefficients--except for the intercept, which is now 50.499. I have no
idea where that number comes from.
On May 10, 2009, at 3:19 PM, Al Best wrote:
center polynomials
Michael Bailey
jm-bailey@northwestern.edu
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