Just a quick question.  Is there a way to easily specify factor levels in a function definition within nls?  For example, I am trying to fit a 3 parameter, nonlinear Weibull function to tree height and I would like to have results by species (or down the road a bit, by plot).  I am hoping there is someway to do it easily, similar to the "gas example" in the ANCOVA chapter in Modern Applied Statistics with S, and not running it with a "by" function or making a very complex function statement with all factor levels combinations.

Can someone point me in the right direction?

Thanks,
Mike

Mike Saunders
Research Assistant
Forest Ecosystem Research Program
Department of Forest Ecosystem Sciences
University of Maine
Orono, ME  04469
207-581-2763 (O)
207-581-4257 (F)

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