Since sending this message I have now solved the problem - needed to alter the
initial values of alpha and beta!
> From: [EMAIL PROTECTED]> To: r-help@r-project.org> Date: Mon, 5 May 2008
> 00:03:53 +0100> Subject: [R] Residual resampling for non linear reg model> >
> > > > I was attempting to use the residual resampling approach to generate
> 999 bootstrap samples of alpha and beta and find their confidence intervals.
> However, I keep getting the error message:Error in nls(resample.mp ~
> cases/(alpha + (beta * cases)), start = init.values, : singular gradientafter
> R has only produced a few bootstraps.Could anyone suggest where I am going
> wrong? Would greatly appreciate it!Mary
> manpower<-read.table("http://www.creem.st-and.ac.uk/len/classes/mt3607/data/manhours_surgical.dat";,
> header=TRUE)#attach dataattach(manpower)B<-999#number of data
> pointsn<-dim(manpower)[1]#alpha level to use for the confidence
> limitsalpha<-0.05#matrix that's going to contain the bootstrap
> coefficientsboot.coef<-matrix(NA, B+1, 2)#fit the initial
> modelinit.values<-c(alpha=20,beta=0)model<-nls(manhours~cases/(alpha+!
(beta*cases)), start=init.values,
trace=TRUE)pred<-predict(model)resid<-resid(model)#do the bootstrapfor (i in
1:B){ #resample the residuals resample.ind!> ex<-sample(1:n,n,replace=T)
resample.mp<-pred+resid[resample.index] #refit the model
init.values<-c(alpha=20,beta=0)
new.model<-nls(resample.mp~cases/(alpha+(beta*cases)),start=init.values,
trace=TRUE) #extract the parameter estimates
boot.coef[i,]<-coef(new.model)}#add the original parameter estimates
boot.coef[B+1,]<-coef(model)#calculate confidence intervalsfor(i in 1:2){
ci<-quantile(boot.coef[,i],probs=c(alpha/2,(1-alpha/2))) cat("residual
bootstrap confidence intervals for parameter",i,"are",ci,"\n")}> > Miss your
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