bartMachine (version 1.2.3)

cov_importance_test: Importance Test for Covariate(s) of Interest

Description

This function tests the null hypothesis $H_0$: These covariates of interest do not affect the response under the assumptions of the BART model.

Usage

cov_importance_test(bart_machine, covariates = NULL, 
num_permutation_samples = 100, plot = TRUE)

Arguments

bart_machine

An object of class ``bart_machine''.

covariates

A vector of names of covariates of interest to be tested for having an effect on the response. A value of NULL indicates an omnibus test for all covariates having an effect on the response. If the name of a covariate is a factor, the entire factor will be permuted. We do not recommend entering the names of factor covariate dummies.

num_permutation_samples

The number of times to permute the covariates of interest and create a corresponding new BART model (see details).

plot

If TRUE, this produces a histogram of the Pseudo-Rsq's / total misclassifcation error rates from the num_permutations BART models created with the covariates permuted. The plot also illustrates the observed Pseudo-Rsq's / total misclassifcation error rate from the original training data and indicates the test's p-value.

Value

permutation_samples_of_error

A vector which records the error metric of the BART models with the covariates permuted (see details).

observed_error_estimate

For regression, this is the Pseudo-Rsq on the original training data set. For classification, this is the observed total misclassification error on the original training data set.

pval

The approximate p-value for this test (see details).

Details

To test the importance of a covariate or a set of covariates of interest on the response, this function generates num_permutations BART models with the covariate(s) of interest permuted (differently each time). On each run, a measure of fit is recorded. For regression, the metric is Pseudo-Rsq; for classification, it is total misclassification error. A p-value can then be generated as follows. For regression, the p-value is the number of permutation-sampled Pseudo-Rsq's greater than the observed Pseudo-Rsq divided by num_permutations + 1. For classification, the p-value is the number of permutation-sampled total misclassification errors less than the observed total misclassification error divided by num_permutations + 1.

References

Adam Kapelner, Justin Bleich (2016). bartMachine: Machine Learning with Bayesian Additive Regression Trees. Journal of Statistical Software, 70(4), 1-40. doi:10.18637/jss.v070.i04

Examples

## Not run: 
# ##regression example
# 
# ##generate Friedman data
# set.seed(11)
# n  = 200 
# p = 5
# X = data.frame(matrix(runif(n * p), ncol = p))
# y = 10 * sin(pi* X[ ,1] * X[,2]) +20 * (X[,3] -.5)^2 + 10 * X[ ,4] + 5 * X[,5] + rnorm(n)
# 
# ##build BART regression model
# bart_machine = bartMachine(X, y)
# 
# ##now test if X[, 1] affects Y nonparametrically under the BART model assumptions
# cov_importance_test(bart_machine, covariates = c(1))
# ## note the plot and the printed p-value
# 
# ## End(Not run)