Dear R community,
Please, can you help me with a problem concerning bootstrap. The data table
called �RMika�, contained times (Tps) and corresponding concentration of a
chemical in a soil (SolA). I would like to get, by bootstraping, 10
estimations of the parameters C0 and k from the function: SolA =
C0*exp(-k*Tps).
# First, I fit the data and all is OK
> tabMika<-read.delim("RMika.txt")
> library(nls)
> attach(tabMika)
> Expon<-function(Tps,parm){
+ C0<-parm[1]
+ k<-parm[2]
+ }
> DegSA.nls<-nls(SolA~C0*exp(-k*Tps),start=c(C0=35, k=1),tabMika)
> summary(DegSA.nls)
Formula: SolA ~ C0 * exp(-k * Tps)
Parameters:
Estimate Std. Error t value Pr(>|t|)
C0 25.682104 1.092113 23.52 < 2e-16 ***
k 0.087356 0.007582 11.52 6.36e-13 ***
---
Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
Residual standard error: 2.584 on 32 degrees of freedom
Correlation of Parameter Estimates:
C0
k 0.7101
# Second, I try to use bootstrap to get the 10 estimates of k and C0
> library(bootstrap)
> theta<-function(tabMika){coef(eval(DegSA.nls$call))}
> bootSolA.nls<-bootstrap(tabMika,10,theta)
Warning message:
multi-argument returns are deprecated in: return(thetastar, func.thetastar,
jack.boot.val, jack.boot.se,
> bootSolA.nls
$thetastar
[,1] [,2] [,3] [,4] [,5] [,6]
C0 25.68210358 25.68210358 25.68210358 25.68210358 25.68210358 25.68210358
k 0.08735615 0.08735615 0.08735615 0.08735615 0.08735615 0.08735615
[,7] [,8] [,9] [,10]
C0 25.68210358 25.68210358 25.68210358 25.68210358
k 0.08735615 0.08735615 0.08735615 0.08735615
# As you can notify, the 10 estimations have the same values for C0 and k.
Moreover, this correspond to the values of k and C0 determined by fitting
all the data without bootstrap!!!???
I cannot find what is wrong. Please, if you find a solution, thank you to
send it to me.
Sincerely
Michael Coeurdassier
Micha�l COEURDASSIER, PhD
Department of Environmental Biology
UC INRA EA3184MRT
Institute for Environmental Sciences and Technology
University of Franche-Comte
Place Leclerc
25030 Besan�on cedex
FRANCE
Tel : +33 (0)381 665 741
Fax : +33 (0)381 665 797
[EMAIL PROTECTED]: [EMAIL PROTECTED]
http://lbe.univ-fcomte.fr/
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