From: Spencer Graves <spencer.graves_at_pdf.com>

Date: Sun 25 Jul 2004 - 00:53:54 EST

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https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Jul 25 01:09:24 2004

Date: Sun 25 Jul 2004 - 00:53:54 EST

I would think that having to eliminate close to 10% of bootstrap samples could be an issue.

Have you considered using

myboot<-boot(Raies,dif.param,R=1000, sim="permutation")

Permutation testing does sampling without replacement rather than sampling with replacement. Perhaps someone else can enlighten us both on the arguments for and against both permutation testing and bootstrapping, but permutation testing could eliminate one source of NAs in the output; in any event, I would want to keep track of the percent of NAs obtained under both. For a statistical audience, you would need to cite literature explaining the difference between permutation testing and bootsrapping, to help people understand your results.

hope this helps. spencer graves

Patrick Giraudoux wrote:

*>Hi,
**>
*

>Just a recap on the trials done based on Spencer Grave, Bervin A Turlach and Christian Ritz's advise.

*>
**>On their suggestion, the trouble mentioned has well been turned using the function try() (using the algorithm "plinear" unstead of
**>"default" was unsuccessful) in the following way:
**>
**>library(boot)
**>
**>dif.param<-function(data,i){
**>RaiesLossA.nls<-try(nls(SolA~a/(1+b*Tps)^c,start=c(a=31,b=0.5,c=0.6),data[i,]),silent=TRUE)
**>RaiesLossB.nls<-try(nls(Solb~a/(1+b*Tps)^c,start=c(a=33,b=1.4,c=0.5),data[i,]),silent=TRUE)
**>
**>if ( (inherits(RaiesLossA.nls,"try-error")) |
**>(inherits(RaiesLossB.nls,"try-error"))) {return(NA)} else {
**>return(RaiesLossA.nls$m$getPars()-RaiesLossB.nls$m$getPars())}
**>
**>}
**>
**>myboot<-boot(Raies,dif.param,R=1000)
**>
**>
**>The boot objet "myboot" that one get cannot however be handled, eg with boot.ci(), because "myboot$t" has NA values. This can be
**>corrected in this way:
**>
**>myboot$t<-na.omit(myboot$t)
**>myboot$R<-length(myboot$t[,1])
**>
**>boot.ci(myboot,index=1, type=c("norm","basic","perc", "bca"))
**>
**>Studentized CI appear to be quite unstable here and were not used.
**>
**>That's it!
**>
**>The effect of omitting some bootstrap replicates (the less than 10% that could not lead to a propper fit) is however questionable:
**>may this
**>biase the result in a way? I am not too much worried about it for my current purpose (crude comparison of parameters), but I
**>guess that it may be an issue for true statisticians...
**>
**>Many thanks for the most helpful hints provided,
**>
**>Patrick Giraudoux
**>
**>
**>
**>
**>
**>
**>>Hi,
**>>
**>>I am trying to bootstrap the difference between each parameters among two non linear regression (distributed loss model) as
**>>following:
**>>
**>># data.frame
**>>
**>>
**>>
**>>
**>>>Raies[1:10,]
**>>>
**>>>
**>>>
**>>>
**>> Tps SolA Solb
**>>1 0 32.97 35.92
**>>2 0 32.01 31.35
**>>3 1 21.73 22.03
**>>4 1 23.73 18.53
**>>5 2 19.68 18.28
**>>6 2 18.56 16.79
**>>7 3 18.79 15.61
**>>8 3 17.60 13.43
**>>9 4 14.83 12.76
**>>10 4 17.33 14.91
**>>etc...
**>>
**>># non linear model (work well)
**>>
**>>RaiesLossA.nls<-nls(SolA~a/(1+b*Tps)^c,start=c(a=32,b=0.5,c=0.6))
**>>RaiesLossB.nls<-nls(Solb~a/(1+b*Tps)^c,start=c(a=33,b=1.5,c=0.5))
**>>
**>>
**>># bootstrap
**>>library(boot)
**>>
**>>dif.param<-function(data,i){
**>>RaiesLossA.nls<-nls(SolA[i]~a/(1+b*Tps[i])^c,start=c(a=31,b=0.5,c=0.6))
**>>RaiesLossB.nls<-nls(Solb[i]~a/(1+b*Tps[i])^c,start=c(a=33,b=1.4,c=0.5))
**>>RaiesLossA.nls$m$getPars()-RaiesLossB.nls$m$getPars()
**>>}
**>>
**>>
**>>
**>>
**>>
**>>>myboot<-boot(Raies,dif.param,R=1000)
**>>>
**>>>
**>>>
**>>>
**>>Error in numericDeriv(form[[3]], names(ind), env) :
**>> Missing value or an Infinity produced when evaluating the model
**>>
**>>It seems that the init values (start=) may come not to be suitable while bootstraping. Data can be sent offline to whom wanted to
**>>try on the dataset.
**>>
**>>Any hint welcome!
**>>
**>>Best regards,
**>>
**>>Patrick Giraudoux
**>>
**>>
**>
**>
**>
**>
**>
**>
**>
**>
*

R-help@stat.math.ethz.ch mailing list

https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Jul 25 01:09:24 2004

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