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The Fowlkes–Mallows index is an external evaluation method that is used to determine the similarity between two clusterings (clusters obtained after a clustering algorithm), and also a metric to measure confusion matrices. This measure of similarity could be either between two hierarchical clusterings or a clustering and a benchmark classification. A higher value for the Fowlkes–Mallows index indicates a greater similarity between the clusters and the benchmark classifications. It was invented by Bell Labs statisticians Edward Fowlkes and Collin Mallows in 1983.[1]
Preliminaries
The Fowlkes–Mallows index, when results of two clustering algorithms are used to evaluate the results, is defined as[2]
where is the number of true positives, is the number of false positives, and is the number of false negatives. is the true positive rate, also called sensitivity or recall, and is the positive predictive rate, also known as precision.
The minimum possible value of the Fowlkes–Mallows index is 0, which corresponds to the worst binary classification possible, where all the elements have been misclassified. And the maximum possible value of the Fowlkes–Mallows index is 1, which corresponds to the best binary classification possible, where all the elements have been perfectly classified.
Definition
Consider two hierarchical clusterings of objects labeled and . The trees and can be cut to produce clusters for each tree (by either selecting clusters at a particular height of the tree or setting different strength of the hierarchical clustering). For each value of , the following table can then be created
where is of objects common between the th cluster of and th cluster of . The Fowlkes–Mallows index for the specific value of is then defined as
where
can then be calculated for every value of and the similarity between the two clusterings can be shown by plotting versus . For each we have .
Fowlkes–Mallows index can also be defined based on the number of points that are common or uncommon in the two hierarchical clusterings. If we define
- as the number of pairs of points that are present in the same cluster in both and .
- as the number of pairs of points that are present in the same cluster in but not in .
- as the number of pairs of points that are present in the same cluster in but not in .
- as the number of pairs of points that are in different clusters in both and .
It can be shown that the four counts have the following property
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