See Andy's previous post on this. -- HTH, Jim Porzak Loyalty Matrix Inc. San Francisco, CA
=============== Liaw, Andy <[EMAIL PROTECTED]> Thu, Oct 27, 2005 at 8:37 AM To: "David L. Van Brunt, Ph.D." <[EMAIL PROTECTED]>, Gabor Grothendieck <[EMAIL PROTECTED]> Cc: [email protected] "classwt" in the current version of the randomForest package doesn't work too well. (It's what was in version 3.x of the original Fortran code by Breiman and Cutler, not the one in the new Fortran code.) I'd advise against using it. "sampsize" and "strata" can be use in conjunction. If "strata" is not specified, the class labels will be used. Take the iris data as an example: randomForest(Species ~ ., iris, sampsize=c(10, 30, 10)) says to randomly draw 10, 30 and 10 from the three species (with replacement) to grow each tree. If you are unsure of the labels, use named vector, e.g., randomForest(Species ~ ., iris, sampsize=c(setosa=10, versicolor=30, virginica=10)) Now, if you want the stratified sampling to be done using a different variable than the class labels; e.g., for multi-centered clinical trial data, you want to draw the same number of patients per center to grow each tree (I'm just making things up, not that that necessarily makes any sense), you can do something like: randomForest(..., strata=center, sampsize=rep(min(table(center))), nlevels(center))) which draws the same number of patients (minimum at any center) from each center to grow each tree. Hope that's clear. Eventually all such things will be in the yet to be written package vignette... Andy On 1/28/07, Betty Health <[EMAIL PROTECTED]> wrote: > Hello there, > > I am working on an extremely unbalanced two class classification problems. I > wanna use "classwt" with "down sampling" together. By checking the rfNews() > in R, it looks that classwt is not working yet. Then I looked at the > software from Salford. I did not find the down sampling option. I am > wondering if you have any experience to deal with this problem. Do you know > any method or softwares can handle this problem? > > Thank you very much!! > > Betty > > [[alternative HTML version deleted]] > > ______________________________________________ > [email protected] 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. > ______________________________________________ [email protected] 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.
