The Rand index measures the quality of the clustering based on the number of True positives and True negatives, with respect to the total number of element pairs.
The TP, TN, FP, TN is computed among the pairs of elements, meaning that a TP happens when a pair of elements is in the same cluster in the ground truth, and is in the same cluster in the final clustering.
The index is defined as:
It measures the level of agreement between the clustering and the ground truth.