Package bootstrap is ORPHANED (that is, unsupported by anyone). Do try
package boot instead.
10 is a very small number of bootstrap replications: I do suggest you try
100 or more.
On Fri, 9 Apr 2004, Micha??l Coeurdassier wrote:
> 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
--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595