[R] Odp: what characteristics of model curve do parameters denote

[Petr PIKAL]

Hi
> 
> 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

No. d Is asymptote. You can easily check how the function behives by

fff<- function(x, a=1,b=1,d=1) d*(1-exp(-b *x))^a

and changing the values of a,b,d,

plot(1:100, fff(1:100, a=..., b=..., d=...)

e.g. 

plot(1:100, fff(1:100, a=1000, b=1,d=10))
points(1:100, fff(1:100, a=1000, b=.1,d=10), col=2)

Regards
Petr


> 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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