node.impurity {oblique.tree}R Documentation

Internal Function to Calculate Impurity of Nodes

Description

Calculates the impurity of a node.

Usage

node.impurity(
        class.probabilities,
        impurity.measure = c("deviance","gini"))

Arguments

class.probabilities A vector of observed class probabilities.
impurity.measure Impurity criterion to use.

Details

Where p_i are observed class probabilities, ``Gini Impurity'' is calculated as

i(p) = sum_{i!=j} p_i p_j = 1-sum_i p_i^2.

The ``deviance'' measure is calculated as

i(p) = -2 sum_i p_i log(p_i)

to allow for internal compatibility. Note: 0 log(0) = 0.

Value

A value for node impurity is returned.

Author(s)

A. Truong

Examples

#A node only has observations of one type
oblique.tree:::node.impurity(
        class.probabilities     = c(1,0,0,0),
        impurity.measure        = "deviance")

#Another node has equal numbers of observations of each type
oblique.tree:::node.impurity(
        class.probabilities     = c(0.25,0.25,0.25,0.25),
        impurity.measure        = "deviance")

[Package oblique.tree version 1.0 Index]