I'm not sure I know the difference between "relative" least squares and "absolute" least squares. Can you explain further?
Whatever it is, the usual procedures have built in objective functions, and I don't know that you can change them. Maybe PROC NLIN would allow you to specify your own objective function.
I have data with high variability
If that's the case, you might want to consider transforming your data so that the errors in your data are closer to normally distributed, rather than "relative" least squares, whatever that is.
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Paige Miller