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