# Rand index

## Description

The Rand index or Rand measure (named after William M. Rand) in statistics, and in particular in data clustering, is a measure of the similarity between two data clusterings. A form of the Rand index may be defined that is adjusted for the chance grouping of elements, this is the adjusted Rand index. From a mathematical standpoint, Rand index is related to the accuracy, but is applicable even when class labels are not used.

Related formulas## Variables

R | Rand index (dimensionless) |

a | the number of pairs of elements in S that are in the same subset in X and in the same subset in Y (dimensionless) |

b | the number of pairs of elements in S that are in different subsets in X and in different subsets in Y (dimensionless) |

n | number of elements (dimensionless) |