The probability of selecting a subtree pair t s t t is calculated by dividing the frequency of the subtree pair in the bag by the number of subtrees that have the same root node labels in this bag.
let t s t t be the number of times the sub-tree pair t s t t occurs in the bag of subtree pairs, and r´tµ be the root node categories of t, then the probability assigned to (1) Given the assumptions that all subtree pairs are independent, the probability of a derivation t s 1 t t 1 AE ¡ ¡ ¡ AE t s N t t N is equal to the product of the probabilities of the used subtree pairs.