This distinguishes DOP1 from most other statistical parsing models that identify exactly one derivation for each parse tree and thus compute the probability of a tree by only one product of probabilities --see Collins (1997<cite> Collins ( , 1999</cite> , Charniak (1997 Charniak ( , 2000 and Eisner (1997 (Dempster et al. 1977) , but this resulted in a decrease in parse accuracy on the ATIS and OVIS corpora (Bod 2000a) , although it slightly improved the word error rate for OVIS word-graphs.