randomForestSRC (version 3.3.1)

synthetic: Synthetic Random Forests

Description

Grows a synthetic random forest (RF) using RF machines as synthetic features. Applies only to regression and classification settings.

Usage

# S3 method for rfsrc
synthetic(formula, data, object, newdata,
  ntree = 1000, mtry = NULL, nodesize = 5, nsplit = 10,
  mtrySeq = NULL, nodesizeSeq = c(1:10,20,30,50,100),
  min.node = 3,
  fast = TRUE,
  use.org.features = TRUE,
  na.action = c("na.omit", "na.impute"),
  oob = TRUE,
  verbose = TRUE,
  ...)

Value

A list with the following components:

rfMachines

RF machines used to construct the synthetic features.

rfSyn

The (grow) synthetic RF built over training data.

rfSynPred

The predict synthetic RF built over test data (if available).

synthetic

List containing the synthetic features.

opt.machine

Optimal machine: RF machine with smallest OOB error rate.

Arguments

formula

Model to be fit. Must be specified unless object is given.

data

Data frame containing the y-outcome and x-variables in the model. Must be specified unless object is given.

object

An object of class (rfsrc, synthetic). Not required when formula and data are supplied.

newdata

Test data used for prediction (optional).

ntree

Number of trees.

mtry

mtry value for over-arching synthetic forest.

nodesize

Nodesize value for over-arching synthetic forest.

nsplit

nsplit-randomized splitting for significantly increased speed.

mtrySeq

Sequence of mtry values used for fitting the collection of RF machines. If NULL, default is number of variables divided by 3, rounded up.

nodesizeSeq

Sequence of nodesize values used for the fitting the collection of RF machines.

min.node

Minimum forest averaged number of nodes a RF machine must exceed in order to be used as a synthetic feature.

fast

Use fast random forests, rfsrc.fast, in place of rfsrc? Improves speed but may be less accurate.

use.org.features

In addition to synthetic features, should the original features be used when fitting synthetic forests?

na.action

Missing value action. The default na.omit removes the entire record if even one of its entries is NA. The action na.impute pre-imputes the data using fast imputation via impute.rfsrc.

oob

Preserve "out-of-bagness" so that error rates and VIMP are honest? Default is yes (oob=TRUE).

verbose

Set to TRUE for verbose output.

...

Further arguments to be passed to the rfsrc function used for fitting the synthetic forest.

Author

Hemant Ishwaran and Udaya B. Kogalur

Details

A collection of random forests are fit using different nodesize values. The predicted values from these machines are then used as synthetic features (called RF machines) to fit a synthetic random forest (the original features are also used in constructing the synthetic forest). Currently only implemented for regression and classification settings (univariate and multivariate).

Synthetic features are calculated using out-of-bag (OOB) data to avoid over-using training data. However, to guarantee that performance values such as error rates and VIMP are honest, bootstrap draws are fixed across all trees used in the construction of the synthetic forest and its synthetic features. The option oob=TRUE ensures that this happens. Change this option at your own peril.

If values for mtrySeq are given, RF machines are constructed for each combination of nodesize and mtry values specified by nodesizeSeq mtrySeq.

References

Ishwaran H. and Malley J.D. (2014). Synthetic learning machines. BioData Mining, 7:28.

See Also

rfsrc, rfsrc.fast

Examples

# \donttest{
## ------------------------------------------------------------
## compare synthetic forests to regular forest (classification)
## ------------------------------------------------------------

## rfsrc and synthetic calls
if (library("mlbench", logical.return = TRUE)) {

  ## simulate the data 
  ring <- data.frame(mlbench.ringnorm(250, 20))

  ## classification forests
  ringRF <- rfsrc(classes ~., ring)

  ## synthetic forests
  ## 1 = nodesize varied
  ## 2 = nodesize/mtry varied
  ringSyn1 <- synthetic(classes ~., ring)
  ringSyn2 <- synthetic(classes ~., ring, mtrySeq = c(1, 10, 20))

  ## test-set performance
  ring.test <- data.frame(mlbench.ringnorm(500, 20))
  pred.ringRF <- predict(ringRF, newdata = ring.test)
  pred.ringSyn1 <- synthetic(object = ringSyn1, newdata = ring.test)$rfSynPred
  pred.ringSyn2 <- synthetic(object = ringSyn2, newdata = ring.test)$rfSynPred


  print(pred.ringRF)
  print(pred.ringSyn1)
  print(pred.ringSyn2)

}

## ------------------------------------------------------------
## compare synthetic forest to regular forest (regression)
## ------------------------------------------------------------

## simulate the data
n <- 250
ntest <- 1000
N <- n + ntest
d <- 50
std <- 0.1
x <- matrix(runif(N * d, -1, 1), ncol = d)
y <- 1 * (x[,1] + x[,4]^3 + x[,9] + sin(x[,12]*x[,18]) + rnorm(n, sd = std)>.38)
dat <- data.frame(x = x, y = y)
test <- (n+1):N

## regression forests
regF <- rfsrc(y ~ ., dat[-test, ], )
pred.regF <- predict(regF, dat[test, ])

## synthetic forests using fast rfsrc
synF1 <- synthetic(y ~ ., dat[-test, ], newdata = dat[test, ])
synF2 <- synthetic(y ~ ., dat[-test, ],
  newdata = dat[test, ], mtrySeq = c(1, 10, 20, 30, 40, 50))

## standardized MSE performance
mse <- c(tail(pred.regF$err.rate, 1),
         tail(synF1$rfSynPred$err.rate, 1),
         tail(synF2$rfSynPred$err.rate, 1)) / var(y[-test])
names(mse) <- c("forest", "synthetic1", "synthetic2")
print(mse)

## ------------------------------------------------------------
## multivariate synthetic forests
## ------------------------------------------------------------

mtcars.new <- mtcars
mtcars.new$cyl <- factor(mtcars.new$cyl)
mtcars.new$carb <- factor(mtcars.new$carb, ordered = TRUE)
trn <- sample(1:nrow(mtcars.new), nrow(mtcars.new)/2)
mvSyn <- synthetic(cbind(carb, mpg, cyl) ~., mtcars.new[trn,])
mvSyn.pred <- synthetic(object = mvSyn, newdata = mtcars.new[-trn,])
# }