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


Michaël COEURDASSIER, PhD
Department of Environmental Biology
UC INRA EA3184MRT
Institute for Environmental Sciences and Technology

University of Franche-Comte
Place Leclerc
25030 Besançon cedex
FRANCE
Tel : +33 (0)381 665 741
Fax : +33 (0)381 665 797

E-m@il: michael.coeurdassier@univ-fcomte.fr
http://lbe.univ-fcomte.fr/

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