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1 wi 1 padd 1 wi wi1 cwi1 v dan jurafsky

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Unformatted text preview: d evidence, •  otherwise bigram, otherwise unigram •  Interpola1on: •  mix unigram, bigram, trigram •  Interpola*on works be_er Dan Jurafsky Linear Interpola1on •  Simple interpola*on •  Lambdas condi*onal on context: Dan Jurafsky How to set the lambdas? •  Use a held ­out corpus Training Data Held ­Out Data Test Data •  Choose λs to maximize the probability of held ­out data: •  Fix the N ­gram probabili*es (on the training data) •  Then search for λs that give largest probability to held ­out set: log P(w1...wn | M (!1...!k )) = " log PM ( !1... !k ) (wi | wi!1 ) i Dan Jurafsky Unknown words: Open versus closed vocabulary tasks •  If we know all the words in advanced •  Vocabulary V is fixed •  Closed vocabulary task •  OIen we don’t know this •  Out Of Vocabulary = OOV words •  Open vocabulary task •  Instead: create an unknown word token <UNK> •  Training of <UNK> probabili*es •  Create a fixed lexicon L of size V •  At text normaliza*on phase, any training word not in L changed to <UNK> •  Now we train its probabili*es like a normal word •  At decoding *me •  If text input: Use UNK probabili*es for any word not in training Dan Jurafsky Huge web ­scale n ­grams •  How to deal with, e.g., Google N ­gram corpus •  Pruning •  Only store N ­grams with count > threshold. •  Remove singletons of higher ­order n ­grams •  Entropy ­based pruning •  Efficiency •  Efficient data structures like tries •  Bloom filters: approximate language models •  Store words as indexes, not strings •  Use Huffman coding to fit large numbers of words into two bytes •  Quan*ze probabili*es (4 ­8 bits instead...
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