Why does the Gini index is used for?
The Gini index is used for decision trees. Indeed, how do we know how to separate the root node? Well, there are a couple of methods, and the Gini index is a good one. It allows checking if the leaves containing labels are pure or impure.
That's right, the more diverse the leaves are, the higher the Gini index is. Why? Because if, let's say, you want to recommend a product using a decision tree, you want to make sure that the leaves are the most homogeneous possible so that you can be confident in your proposition.

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