problem with nls starting values

thanks for your reply

I agree that an lm model would fit just as well, however the expectation from a mechanistic point of view would be a non-linear relationship.

Also when I "simulate" data as in

y_val<-115-118*exp(-0.12*(seq(1,100)+rnorm(100,0,0.8)))
x_val<-seq(1:100)
plot(y_val~x_val)
summary(mod1<-nls(y_val~a-b*exp(-c*x_val),start=list(a=115,b=118,c=0.12)))

I do not get a convergence. I obviously must be doing something wrong.

I do get convergence:

nls(y_val~a-b*exp(-c*x_val),start=list(a=115,b=118,c=0.12))

Nonlinear regression model
  model:  y_val ~ a - b * exp(-c * x_val) 
   data:  parent.frame() 
       a        b        c 
115.0420 117.0529   0.1192 
 residual sum-of-squares: 181.6

Number of iterations to convergence: 3 
Achieved convergence tolerance: 1.436e-07

sessionInfo()

R version 2.15.1 Patched (2012-09-11 r60679)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

locale:
[1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     


Berend