Shannon Entropy

Gini index 와 비슷하게, 비균등 분포를 측정할 때 사용할 수 있는 지표

$$H=-\sum _{i=1}^{n}p(i)\log p(i)$$

The entropy is $0$ when a single item is always chosen or recommended, and $\log n$ when $n$ items are chosen or recommended equally often.