The accuracy of the word-word dependencies extracted from the PTB-CFG trees converted from the HPSG treebank was 98.76%.
we assign small scores to the back-off rules as: where is a small constant, P (X → α) is the maximum-likelihood estimate of the PCFG rule probability of X → α in the source treebank, and is the product of 're-labeling probabilities' for the pairs of linked non-terminal symbols in α and β, defined as: where Count(•) represents the number of commonspan node pairs used as the split points.