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Fitting Different Slopes Common Intercepts

Feb 29, 2020 3:03 AM
(1451 views)

Hi - i am unsure how to fit a regression model with different slopes BUT a common Interecpt.

Assume Y=response, GROUP (2-levels), TIme (Covariate)

Kind regards, Vince

12 REPLIES 12

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You can do this in Fit Y by X: Y=response, X=time. After the plot, Group by GROUP. Then use Fit Special and constrain the intercept to 0.

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Re: Fitting Different Slopes Common Intercepts

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Re: Fitting Different Slopes Common Intercepts

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Re: Fitting Different Slopes Common Intercepts

If you also want to estimate the common intercept, try this approach.

- Select
**Analyze > Fit Model**. - Click the red triangle at the top left and de-select
**Center Polynomials**. - Select the
**response**column and click**Y**. - Select the
**time and group**columns and click**Macros**, then select**Factorial to Degree**. - Select the
**group effect**and click**Remove**. - JMP will ask you to if it should remove all terms with this column. Click
**No**. - Click
**Run**. - JMP will warn you about a missing effect. Click
**Continue**.

In general, we would keep the group effect in the model to maintain the *model hierarchy*. But that leads to a unique or different intercept for each group. This way you will get estimates and tests for the common intercept and unique group slopes.

Learn it once, use it forever!

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Re: Fitting Different Slopes Common Intercepts

Created:
Feb 29, 2020 11:25 AM
| Last Modified: Feb 29, 2020 12:45 PM
(1418 views)
| Posted in reply to message from markbailey 02-29-2020

Thanks! I made up 40 because it looked good (better than zero, anyway).

edit: adding an image. Left is fit y by x, center is fit model. Right is illustrating how to get the slopes from fit model.

Craige

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Re: Fitting Different Slopes Common Intercepts

Thanks...curious to knnow if there is an alternative way without this to stage process..

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Re: Fitting Different Slopes Common Intercepts

Fit model does give the intercept and the two slopes in a single step. The picture is comparing the easy-to-read output in fit y by x with the parameters returned by fit model. I puzzled it out by comparing them; I hope @markbailey is watching in case I've misinterpreted something.

Craige

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Re: Fitting Different Slopes Common Intercepts

Thanks - how did you estimate the common intercept using fit y by x of 49?.

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Re: Fitting Different Slopes Common Intercepts

I copied it from fit model to @dale_lehman answer, instead of the zero he suggested or the 40 I initially guessed. If you start with fit model rather than fit y by x, you get parameters for both lines without copying the numbers.

Craige

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Re: Fitting Different Slopes Common Intercepts

Thanks - so I assume fit model implementation is required to get the common intercept and slopes first then fit by x to get the figure. Thanks for your help . Appreciate it

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