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Sep 18, 2013 1:58 PM
(1141 views)

1 ACCEPTED SOLUTION

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Solution

In the model dialog for the 2nd degree polynomial with the linear term removed, click the red triangle next to Model Specification and uncheck "Center Polynomials". Then your results will agree with the other model you made. The model predictions will be the same whether centered or not.

Michael

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In the model dialog for the 2nd degree polynomial with the linear term removed, click the red triangle next to Model Specification and uncheck "Center Polynomials". Then your results will agree with the other model you made. The model predictions will be the same whether centered or not.

Michael

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Sep 27, 2014 5:16 AM
(888 views)

I like your answer, but I have wondered: WHY center the polynomials?

Steve

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Sep 27, 2014 11:49 AM
(888 views)

If you search the help index for "Center Polynomials" you will find this:

Causes any continuous term involved in an effect with degree greater than one to be centered by its mean. This option is checked by default, except when a term involved in the effect is assigned the Mixture Effect attribute or has the Mixture column property. Terms with the Coding column property are centered midway between their specified High and Low values.

**Centering is useful is making regression coefficients more interpretable and in reducing collinearity between model effects**. (my emphasis)

If you search Google for "Center Variables" you can find additional discussion on this topic.

Michael