[R] what characteristics of model curve do parameters denote

[Anna Lee]

Dear list,

I'm trying to fit a chapman-richards equation to my data, only I
cannot interpret the parameters a, b and d. I know that the parameter
b denotes the asymptote, but for the others I couldn't figure out. But
I do need to know this in order to set my starting values. Here's the
model:

modPoplar<- nls(Diameter ~ d*(1-exp(-b *Age))^a ,start=list(a=20,b=0.9,d=33))

I attached the graph, too.

Hoping for your answers!

Best, Anna

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