The probability of a parse tree is computed by summing up the probabilities of all derivations producing it, while the probability of a derivation is computed by multiplying the (smoothed) relative frequencies of its subtrees.
the probability of a subtree t is taken as the number of occurrences of t in the binary tree-set, | t |, divided by the total number of occurrences of all subtrees t' with the same root label as t. Let r(t) return the root label of t: The subtree probabilities are smoothed by applying simple Good-Turing to the subtree distribution (see Bod 1998: 85-87).