Given for example two classifications (C and K) of n residues, it is possible to count the number of cases in which residues i and j were classified in the same group in C and K (n_ss), the number of cases in which i and j were classified in the same group in C and in different groups in K (n_sd), the number of cases in which i and j were classified into two different groups in C and in the same group in K (n_ds), and the number of cases in which i and j were classified into two different groups both in C and in K (n_dd). On the basis of this description, it is possible to compute the Jaccard index (J), the Rand coefficient (R), and the Fowlkes-Mallows index (FM), which are defined as: