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Level II

## How to see equation used for spline?

So I'm using the XY fit to generate a spline to fit a curve. See simple example attached. How do I get to the equations JMP's used to generate the curve please?

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Staff (Retired)

## Re: How to see equation used for spline?

Also see @Duane_Hayes'  article Understanding cubic splines

Craige
5 REPLIES 5
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Staff

## Re: How to see equation used for spline?

I believe that Bivariate and Graph Builder will fit spline models but neither one will save the model where you can see or use it outside of the platform. The Fit Least Squares platform can fit, profile, and save spline models. They work differently. (There are different kinds of spline models.) Select Analyze > Fit Model and select your response and predictor that you used in Bivariate. Select the predictor in the Effects list, click the red triangle next to Attributes, and select Spline Effect. Click Run and the select Save Columns > Prediction Formula.

Note that Fit Model will ask you to specify the number of knots instead of specifying the stiffness parameter Lambda as in Bivariate.

Learn it once, use it forever!
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Level II

## Re: How to see equation used for spline?

Thanks for the quick reply Mark. Can't quite see how to generate the spline curve that I could in Fit XY.

I can find the Fit Model dialogue you mention. I've put Y as a Role Variable, X as a Construct Model Effect. I can see an option to update the Degree (which default is '2'). I've left Personality as 'Standard Least Squares', Emphasis as 'Effect Leverage', and kept Weight, Freq, Validation, By all blank.

When I select "Knotted Spline Effect" in Attributes, nothing happens to ask me about number of knots.

I've saved the scripts for the spline and the fit to the attached data table.

The output seems to just fit a straight line.

What am I missing?

Cheers

John

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Level II

## Re: How to see equation used for spline?

Ahhh! If you click on X so it's selected as a Construct Model Effect and THEN click on attributes and choose Knotted Spline Effect, then the option to choose number of knots appears. Will keep digging, but that need to selectd the effect first wasn't clear to this newbie...

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Staff

## Re: How to see equation used for spline?

Yes, that changes the modeling of the effect of your X. Let me know if you have any more questions.

Learn it once, use it forever!
Highlighted
Staff (Retired)

## Re: How to see equation used for spline?

Also see @Duane_Hayes'  article Understanding cubic splines

Craige
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