[R] bootstrapping - number of items to replace is not a multiple of replacement length

[Gabriela Bucini]

HI Steve,

I've checked the random forest package. The VariableImportance does exactly
what I need.
Thank you so much!

Gabriela


> -----Original Message-----
> From: Steven McKinney [mailto:smckinney at bccrc.ca]
> Sent: Tuesday, September 09, 2008 8:48 PM
> To: Gabriela Bucini; r-help at r-project.org
> Subject: RE: [R] bootstrapping - number of items to replace is not a
> multiple of replacement length
>
>
> Hello,
>
> Your theta() function is returning different
> sets of coefficients depending on the results of
> step().
>
> You'll need to add code to theta() to figure
> out which variables were selected, and store
> them into the right positions of a vector
> of length 20 (the apparent number of covariates
> you describe below), so that your theta()
> function always returns the same sized output.
>
> (Google
>
>    stepwise regression random forest
>
> and you'll get a number of hits
> about using random forests instead
> of stepwise regression, and pointers
> about bootstrapping the random forest.)
>
> HTH
>
> Steve McKinney
>
>
> > -----Original Message-----
> > From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org]
> > On Behalf Of Gabriela Bucini
> > Sent: Tuesday, September 09, 2008 5:02 PM
> > To: r-help at r-project.org
> > Subject: [R] bootstrapping - number of items to replace is not a
> multiple
> > ofreplacement length
> >
> > Hello,
> >
> > I'm new to boostrapping and I'd need some help to understand the error
> > message that pops up when I run my script.
> >
> > I have a data.frame with 73 lines and 21 column.
> > I am running a stepwise regression to find the best model using the R
> > function "step".
> > I apply bootstrapping to obtain model coefficients.
> > This is my script:
> >
> > # "datare80" is the name of the data.frame and "woodycover" is the
> > response
> > variable
> > theta <- function(datare80, indices) {
> >              d <- datare80[indices, ]    # allows boot to select
> subsample
> > datasets
> >              full <- lm(d$woodycover~ ., data= d )
> >              lmbroadst <- step(full, data=d , direction = "both", k=2,
> > trace=0)
> >              coefficients(lmbroadst)  # return coef. vector
> >           }
> > resb <- boot(data = datare80, statistic = theta, R=1000)
> >
> >
> >
> > When I run it, I get these two messages:
> > If I omit the last line "coefficients(lmbroadst)" in the function
> > definition, I get :
> > "Error in t.star[r, ] <- statistic(data, i[r, ], ...) :
> >         incorrect number of subscripts on matrix"
> > If I have the last line "coefficients(lmbroadst)", then I get:
> > "Error in t.star[r, ] <- statistic(data, i[r, ], ...) :
> >         number of items to replace is not a multiple of replacement
> > length"
> >
> >
> > Thank you very much for any help!
> >
> > Gabriela
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/posting-
> > guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
>