Local linear forest — ll_regression

Trains a local linear forest that can be used to estimate the conditional mean function mu(x) = E[Y | X = x]

ll_regression_forest(
  X,
  Y,
  enable.ll.split = FALSE,
  ll.split.weight.penalty = FALSE,
  ll.split.lambda = 0.1,
  ll.split.variables = NULL,
  ll.split.cutoff = NULL,
  num.trees = 2000,
  clusters = NULL,
  equalize.cluster.weights = FALSE,
  sample.fraction = 0.5,
  mtry = min(ceiling(sqrt(ncol(X)) + 20), ncol(X)),
  min.node.size = 5,
  honesty = TRUE,
  honesty.fraction = 0.5,
  honesty.prune.leaves = TRUE,
  alpha = 0.05,
  imbalance.penalty = 0,
  ci.group.size = 2,
  tune.parameters = "none",
  tune.num.trees = 50,
  tune.num.reps = 100,
  tune.num.draws = 1000,
  num.threads = NULL,
  seed = runif(1, 0, .Machine$integer.max)
)

Arguments

X	The covariates used in the regression.
Y	The outcome.
enable.ll.split	(experimental) Optional choice to make forest splits based on ridge residuals as opposed to standard CART splits. Defaults to FALSE.
ll.split.weight.penalty	If using local linear splits, user can specify whether or not to use a covariance ridge penalty, analogously to the prediction case. Defaults to FALSE.
ll.split.lambda	Ridge penalty for splitting. Defaults to 0.1.
ll.split.variables	Linear correction variables for splitting. Defaults to all variables.
ll.split.cutoff	Enables the option to use regression coefficients from the full dataset for LL splitting once leaves get sufficiently small. Leaf size after which we use the overall beta. Defaults to the square root of the number of samples. If desired, users can enforce no regulation (i.e., using the leaf betas at each step) by setting this parameter to zero.
num.trees	Number of trees grown in the forest. Note: Getting accurate confidence intervals generally requires more trees than getting accurate predictions. Default is 2000.
clusters	Vector of integers or factors specifying which cluster each observation corresponds to. Default is NULL (ignored).
equalize.cluster.weights	If FALSE, each unit is given the same weight (so that bigger clusters get more weight). If TRUE, each cluster is given equal weight in the forest. In this case, during training, each tree uses the same number of observations from each drawn cluster: If the smallest cluster has K units, then when we sample a cluster during training, we only give a random K elements of the cluster to the tree-growing procedure. When estimating average treatment effects, each observation is given weight 1/cluster size, so that the total weight of each cluster is the same. Default is FALSE.
sample.fraction	Fraction of the data used to build each tree. Note: If honesty = TRUE, these subsamples will further be cut by a factor of honesty.fraction. Default is 0.5.
mtry	Number of variables tried for each split. Default is \(\sqrt p + 20\) where p is the number of variables.
min.node.size	A target for the minimum number of observations in each tree leaf. Note that nodes with size smaller than min.node.size can occur, as in the original randomForest package. Default is 5.
honesty	Whether to use honest splitting (i.e., sub-sample splitting). Default is TRUE. For a detailed description of honesty, honesty.fraction, honesty.prune.leaves, and recommendations for parameter tuning, see the grf algorithm reference.
honesty.fraction	The fraction of data that will be used for determining splits if honesty = TRUE. Corresponds to set J1 in the notation of the paper. Default is 0.5 (i.e. half of the data is used for determining splits).
honesty.prune.leaves	If TRUE, prunes the estimation sample tree such that no leaves are empty. If FALSE, keep the same tree as determined in the splits sample (if an empty leave is encountered, that tree is skipped and does not contribute to the estimate). Setting this to FALSE may improve performance on small/marginally powered data, but requires more trees (note: tuning does not adjust the number of trees). Only applies if honesty is enabled. Default is TRUE.
alpha	A tuning parameter that controls the maximum imbalance of a split. Default is 0.05.
imbalance.penalty	A tuning parameter that controls how harshly imbalanced splits are penalized. Default is 0.
ci.group.size	The forest will grow ci.group.size trees on each subsample. In order to provide confidence intervals, ci.group.size must be at least 2. Default is 1.
tune.parameters	If true, NULL parameters are tuned by cross-validation; if FALSE NULL parameters are set to defaults. Default is FALSE. Currently, local linear tuning is based on regression forest fit, and is only supported for `enable.ll.split = FALSE`.
tune.num.trees	The number of trees in each 'mini forest' used to fit the tuning model. Default is 10.
tune.num.reps	The number of forests used to fit the tuning model. Default is 100.
tune.num.draws	The number of random parameter values considered when using the model to select the optimal parameters. Default is 1000.
num.threads	Number of threads used in training. By default, the number of threads is set to the maximum hardware concurrency.
seed	The seed of the C++ random number generator.

Value

A trained local linear forest object.

Examples

# \donttest{
# Train a standard regression forest.
n <- 50
p <- 10
X <- matrix(rnorm(n * p), n, p)
Y <- X[, 1] * rnorm(n)
forest <- ll_regression_forest(X, Y)
# }