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Introduced on day 14

Adjusted Rand index (ARI)

A measure of agreement between two labellings of the same rows that is 1 for identical partitions and about 0 for chance-level agreement.

In detail

ARI counts pairs of rows placed together or apart in both labellings and corrects for the agreement expected by chance, so it ignores label names and numbering. It compares two clusterings (stability) or a clustering with known groups (validation in simulations). Values can be slightly negative when agreement is worse than chance.