The dissertation of Andreas Stolcke is approved: University of ...

, u1 ¸¼u Ù 0 and , u3 ¸¹u Ù 0&6|model variant due to merging is now possible in ðð, u1 ¸¼u Ù 0MN¬,,, u2 ¸¹u Ù 0F£,CHAPTER 3. HIDDEN MARKOV MODELS 44, u2 ¸¹u ÙY0 in the current model, the merged state u 3 would be assigned a countlikely paths in the merged model, simply replacing the merged states with u 3, and no other samples changetheir paths to include u 3 ¸¼u Ù .This path preservation assumption is not strictly true but holds most of the time, since the mergesactually chosen are those that collapse states with similar distributionsof transition and emission probabilities.The assumption can be easily tested, and the counts corrected, by reparsing the training data from time totime.In an incremental model building scenario, where new samples are available in large number andincorporated one by one, interleaved with merging, one might not want to store all data seen in the past. Inthis case an exponentially decaying average of Viterbi counts can be kept instead. This has the effect thatincorrect Viterbi counts will eventually fade away, being replaced by up-to-date counts obtained form parsingmore recent data with the current model.This is correct if all samples with Viterbi paths through the transitions u 1 ¸¹u and u 2 ¸u Ù retain their mostIncremental model evaluationUsing the techniques described in the previous sections, the evaluation of a. 3§. 0 amortized time, instead of the ð .ñÞ9.N . 3§. 0