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on here appreciating this older tip about using Fit Model by not centering polynomials and just thought I'd note that in Fit Model the Expanded Estimates are a little less cryptic, especially if there were more groups. I'll be using those where I want to get the slopes out of there with JSL but can see that the plot from Fit Y by X is nicer.
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.
Here is another approach, using the Nonlinear platform. Attached is an example: you need to set parameters to be solved for, with a single intercept but varying slope parameters for the two groups. The script is saved for the solution.