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May 24, 2017 7:01 PM
(871 views)

I have a pretty basic question that I cannot find the solution to....

When performing a polynomial fit as shown below...

Is there a way to get the resulting polynomial in the typical form like that below?

y = c0 + c1*x + c2*x^2

Im sure there is a clever reason for giving fit results in the JMP default form...please enlighten me as to why.

Thx,

Rory

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May 25, 2017 5:06 AM
(980 views)

Solution

JMP automatically centers variables when higher order terms are added to the model. This practice is recommended to reduce the collinearity among predictors to, in turn, reduce the variance inflation. (x^2 is highly collinear with x otherwise.)

There are two ways to get the form that you want:

**Fit Using the Uncentered Variable**: Click the red triangle next to**Bivariate Fit**and select**Fit Special**. The option to center variables is near the bottom. Here you can also select the second-order polynomial model to be fit.**Convert the Model Afterward**: Save the equation as the prediction formula. Open the formula, click the red triangle and select**Simplify**.

Hope one of these ways is satisfactory.

Learn it once, use it forever!

1 REPLY

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May 25, 2017 5:06 AM
(981 views)

JMP automatically centers variables when higher order terms are added to the model. This practice is recommended to reduce the collinearity among predictors to, in turn, reduce the variance inflation. (x^2 is highly collinear with x otherwise.)

There are two ways to get the form that you want:

**Fit Using the Uncentered Variable**: Click the red triangle next to**Bivariate Fit**and select**Fit Special**. The option to center variables is near the bottom. Here you can also select the second-order polynomial model to be fit.**Convert the Model Afterward**: Save the equation as the prediction formula. Open the formula, click the red triangle and select**Simplify**.

Hope one of these ways is satisfactory.

Learn it once, use it forever!