Information Gain
In decision tree analyses, an important concept is the gain ratio from a parent node to its children.
The gain ratio, ∆, measures the gain in purity from parent to children, weighted by the relative size of the subsets, as follows.
where I is the purity (or impurity) of a node;
N is the number of elements assigned to child node j;
N is the total number of elements at the parent node.
The decision tree algorithm tries to perform a splitting that maximizes this gain ratio.
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