  We investigate how asymmetrizing an impurity function affects the choice of
optimal node splits when growing a decision tree for binary classification. In
particular, we relax the usual axioms of an impurity function and show how
skewing an impurity function biases the optimal splits to isolate points of a
particular class when splitting a node. We give a rigorous definition of this
notion, then give a necessary and sufficient condition for such a bias to hold.
We also show that the technique of class weighting is equivalent to applying a
specific transformation to the impurity function, and tie all these notions
together for a class of impurity functions that includes the entropy and Gini
impurity. We also briefly discuss cost-insensitive impurity functions and give
a characterization of such functions.
