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