The probability of a parse tree is computed from the occurrencefrequencies of the subtrees in the treebank.
the probability of a subtree t is taken as the number of occurrences of t in the training set, I t I, 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 probability of a derivation ti0...0t n is computed by the product of the probabilities of its subtrees ti: An important feature of DOP1 is that there may be several derivations that generate the same parse tree.