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I am interested in testing two similar nls models to determine if the lines are statistically different when fitted with two different data sets; one corn, another soybean. I know I can do this in linear models by testing for interactions. See Introductory Statistics with R by Dallgaard p212-218 for an example. I have two different data sets I am comparing to lai. ci.re should have very little difference between corn and soybean, while ci.gr, there should be a difference. If I use the simplistic form described in Dallgaard (see example in code below) it does not work correctly...