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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > On Behalf Of Gabriela Bucini > Sent: Tuesday, September 09, 2008 5:02 PM > To: r-help@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@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. ______________________________________________ R-help@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.
