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Fitting Different Slopes Common Intercepts

vincem

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

Accepted answers

All comments

dale_lehman

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.

Craige_Hales

@dale_lehman Cool!

Male and Female groups have same intercept

Mark_Bailey

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.

Craige_Hales

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.

@Mark_Bailey

Getting the slopes from Fit Model

vincem

Thansk...however when i adopt the below approach the graph of Y verus TIME by group - doesnt appear to haev a common intercept. Howevre if i change Time to an ordinal variable..seems to work OK.