[R] Generating a best fit line for non linear data

[Andrew Robinson]

I think that you probably need to provide the x values to nls. Try,
for example,


fit <- nls(species ~ a *(1 - exp(-b*samples)),start = list(a = 27, b = .15))


I hope that this helps,

Andrew

On Thu, Apr 28, 2011 at 01:56:43PM -0700, BornSurvivor wrote:
> I have the following data set, and I have to find the line of best fit using
> this equation, 
> y = a*(1 - exp(-b*x)).
> 
> samples = seq(1,20,by=1)
> species = c(5,8,9,9,11,11,12,15,17,19,20,20,21,23,23,25,25,27,27,27)
> plot(samples,species, main = "Accumulation Curve for Tree Species Richness",
> xlab = "Samples", ylab = "Number of Species")
> curve((y = 27*(1 - exp(-.15*x))),from=0,to=20,add = T)
> 
> I tried to find the best fit curve using:
> 
> fit = nls(species ~ a *(1 - exp(-b*x)),start = list(a = 27, b = .15)
> 
> but I get a "parameters without starting value in 'data': x" and I don't
> have any idea what this means, or how to fix the above code.  
> 
> --
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-- 
Andrew Robinson  
Program Manager, ACERA 
Department of Mathematics and Statistics            Tel: +61-3-8344-6410
University of Melbourne, VIC 3010 Australia               (prefer email)
http://www.ms.unimelb.edu.au/~andrewpr              Fax: +61-3-8344-4599
http://www.acera.unimelb.edu.au/

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