The Jaccard index, also known as the Jaccard similarity coefficient, is a statistic used for measuring the similarity between two sets. It is defined as the size of the intersection of the sets divided by the size of the union of the sets.
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The index measures similarity of finite sets by shared presence relative to total presence. For binary profiles it ignores joint absences, unlike the simple matching coefficient. When both sets are empty, define the similarity to be one. The complementary Jaccard distance measures dissimilarity.