Repost: Examples of "classwt", "strata", and "sampsize" i n randomForest?

"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

Repost: Examples of "classwt", "strata", and "sampsize" i n randomForest?

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