The total gini impurity of a tree is the weighted average of gini impurities (weighted on samples) of the leaves. These leaves are weighted as they may not represent the same number of samples.

Example:

flowchart TD A[Loves popcorn] C[Does not love the song\nSamples: 100\nGini impurity: 0.3] D[Loves cats] E[Loves the song\nSamples: 50\nGini impurity: 0.1] F[Does not love the song\nSamples: 10\nGini impurity: 0.5] A --> |True| D A --> |False| C D --> |True| E D --> |False| F

$\text{total gini impurity} = \frac{100}{160}\times0.3 + \frac{50}{160}\times0.1 + \frac{10}{160}\times0.5 = 0.25$

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