#### Transcript LSA.303 Introduction to Computational Linguistics

LSA 352: Speech Recognition and Synthesis Dan Jurafsky Lecture 1: 1) Overview of Course 2) Refresher: Intro to Probability 3) Language Modeling IP notice: some slides for today from: Josh Goodman, Dan Klein, Bonnie Dorr, Julia Hirschberg, Sandiway Fong LSA 352 Summer 2007 1 Outline Overview of Course Probability Language Modeling Language Modeling means “probabilistic grammar” LSA 352 Summer 2007 2 Definitions Speech Recognition Speech-to-Text – Input: a wavefile, – Output: string of words Speech Synthesis Text-to-Speech – Input: a string of words – Output: a wavefile LSA 352 Summer 2007 3 Automatic Speech Recognition (ASR) Automatic Speech Understanding (ASU) Applications Dictation Telephone-based Information (directions, air travel, banking, etc) Hands-free (in car) Second language ('L2') (accent reduction) Audio archive searching Linguistic research – Automatically computing word durations, etc LSA 352 Summer 2007 4 Applications of Speech Synthesis/Text-to-Speech (TTS) Games Telephone-based Information (directions, air travel, banking, etc) Eyes-free (in car) Reading/speaking for disabled Education: Reading tutors Education: L2 learning LSA 352 Summer 2007 5 Applications of Speaker/Lg Recognition Language recognition for call routing Speaker Recognition: Speaker verification (binary decision) – Voice password, telephone assistant Speaker identification (one of N) – Criminal investigation LSA 352 Summer 2007 6 History: foundational insights 1900s-1950s Automaton: Markov 1911 Turing 1936 McCulloch-Pitts neuron (1943) – http://marr.bsee.swin.edu.au/~dtl/het704/lecture10/ann/node 1.html – http://diwww.epfl.ch/mantra/tutorial/english/mcpits/html/ Shannon (1948) link between automata and Markov models Human speech processing Fletcher at Bell Labs (1920’s) Probabilistic/Information-theoretic models Shannon (1948) LSA 352 Summer 2007 7 Synthesis precursors Von Kempelen mechanical (bellows, reeds) speech production simulacrum 1929 Channel vocoder (Dudley) LSA 352 Summer 2007 8 History: Early Recognition • 1920’s Radio Rex QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Celluloid dog with iron base held within house by electromagnet against force of spring Current to magnet flowed through bridge which was sensitive to energy at 500 Hz 500 Hz energy caused bridge to vibrate, interrupting current, making dog spring forward The sound “e” (ARPAbet [eh]) in Rex has 500 Hz component LSA 352 Summer 2007 9 History: early ASR systems • 1950’s: Early Speech recognizers 1952: Bell Labs single-speaker digit recognizer – Measured energy from two bands (formants) – Built with analog electrical components – 2% error rate for single speaker, isolated digits 1958: Dudley built classifier that used continuous spectrum rather than just formants 1959: Denes ASR combining grammar and acoustic probability 1960’s FFT - Fast Fourier transform (Cooley and Tukey 1965) LPC - linear prediction (1968) 1969 John Pierce letter “Whither Speech Recognition?” – Random tuning of parameters, – Lack of scientific rigor, no evaluation metrics – Need to rely on higher level knowledge LSA 352 Summer 2007 10 ASR: 1970’s and 1980’s Hidden Markov Model 1972 Independent application of Baker (CMU) and Jelinek/Bahl/Mercer lab (IBM) following work of Baum and colleagues at IDA ARPA project 1971-1976 5-year speech understanding project: 1000 word vocab, continous speech, multi-speaker SDC, CMU, BBN Only 1 CMU system achieved goal 1980’s+ Annual ARPA “Bakeoffs” Large corpus collection – TIMIT – Resource Management – Wall Street Journal LSA 352 Summer 2007 11 State of the Art ASR speaker-independent, continuous, no noise, world’s best research systems: – Human-human speech: ~13-20% Word Error Rate (WER) – Human-machine speech: ~3-5% WER TTS (demo next week) LSA 352 Summer 2007 12 LVCSR Overview Large Vocabulary Continuous (Speaker-Independent) Speech Recognition Build a statistical model of the speech-to-words process Collect lots of speech and transcribe all the words Train the model on the labeled speech Paradigm: Supervised Machine Learning + Search LSA 352 Summer 2007 13 Unit Selection TTS Overview Collect lots of speech (5-50 hours) from one speaker, transcribe very carefully, all the syllables and phones and whatnot To synthesize a sentence, patch together syllables and phones from the training data. Paradigm: search LSA 352 Summer 2007 14 Requirements and Grading Readings: Required Text: Selected chapters on web from – Jurafsky & Martin, 2000. Speech and Language Processing. – Taylor, Paul. 2007. Text-to-Speech Synthesis. Grading Homework: 75% (3 homeworks, 25% each) Participation: 25% You may work in groups LSA 352 Summer 2007 15 Overview of the course http://nlp.stanford.edu/courses/lsa352/ LSA 352 Summer 2007 16 6. Introduction to Probability Experiment (trial) Repeatable procedure with well-defined possible outcomes Sample Space (S) – the set of all possible outcomes – finite or infinite Example – coin toss experiment – possible outcomes: S = {heads, tails} Example – die toss experiment – possible outcomes: S = {1,2,3,4,5,6} Qui ckT ime™ and a T IFF (Uncompres sed) dec ompres sor are needed to s ee this pic ture. Quic kT ime™ and a T IFF (Uncompress ed) decompress or are needed to s ee this pi cture. LSA 352 Summer 2007 Slides from Sandiway Fong 17 Introduction to Probability Definition of sample space depends on what we are asking Sample Space (S): the set of all possible outcomes Example – die toss experiment for whether the number is even or odd – possible outcomes: {even,odd} – not {1,2,3,4,5,6} Quic kT ime™ and a T IFF (Uncompress ed) decompress or are needed to s ee this pi cture. LSA 352 Summer 2007 18 More definitions Events an event is any subset of outcomes from the sample space Example die toss experiment let A represent the event such that the outcome of the die toss experiment is divisible by 3 A = {3,6} A is a subset of the sample space S= {1,2,3,4,5,6} Example Draw a card from a deck – suppose sample space S = {heart,spade,club,diamond} (four suits) let A represent the event of drawing a heart let B represent the event of drawing a red card A = {heart} B = {heart,diamond} QuickTime™and a TIFF(Uncompressed) decompressor are needed to see this pi cture. LSA 352 Summer 2007 19 Introduction to Probability Some definitions Counting – suppose operation oi can be performed in ni ways, then – a sequence of k operations o1o2...ok – can be performed in n1 n2 ... nk ways Example – die toss experiment, 6 possible outcomes – two dice are thrown at the same time – number of sample points in sample space = 6 6 = 36 Quic kT ime™ and a T IFF (Uncompress ed) dec ompress or are needed to s ee this pic ture. LSA 352 Summer 2007 20 Definition of Probability The probability law assigns to an event a nonnegative number Called P(A) Also called the probability A That encodes our knowledge or belief about the collective likelihood of all the elements of A Probability law must satisfy certain properties LSA 352 Summer 2007 21 Probability Axioms Nonnegativity P(A) >= 0, for every event A Additivity If A and B are two disjoint events, then the probability of their union satisfies: P(A U B) = P(A) + P(B) Normalization The probability of the entire sample space S is equal to 1, I.e. P(S) = 1. LSA 352 Summer 2007 22 An example An experiment involving a single coin toss There are two possible outcomes, H and T Sample space S is {H,T} If coin is fair, should assign equal probabilities to 2 outcomes Since they have to sum to 1 P({H}) = 0.5 P({T}) = 0.5 P({H,T}) = P({H})+P({T}) = 1.0 LSA 352 Summer 2007 23 Another example Experiment involving 3 coin tosses Outcome is a 3-long string of H or T S ={HHH,HHT,HTH,HTT,THH,THT,TTH,TTTT} Assume each outcome is equiprobable “Uniform distribution” What is probability of the event that exactly 2 heads occur? A = {HHT,HTH,THH} P(A) = P({HHT})+P({HTH})+P({THH}) = 1/8 + 1/8 + 1/8 =3/8 LSA 352 Summer 2007 24 Probability definitions In summary: Probability of drawing a spade from 52 well-shuffled playing cards: LSA 352 Summer 2007 25 Probabilities of two events If two events A and B are independent Then P(A and B) = P(A) x P(B) If flip a fair coin twice What is the probability that they are both heads? If draw a card from a deck, then put it back, draw a card from the deck again What is the probability that both drawn cards are hearts? A coin is flipped twice What is the probability that it comes up heads both times? LSA 352 Summer 2007 26 How about non-uniform probabilities? An example A biased coin, twice as likely to come up tails as heads, is tossed twice What is the probability that at least one head occurs? Sample space = {hh, ht, th, tt} (h = heads, t = tails) Sample points/probability for the event: ht 1/3 x 2/3 = 2/9 th 2/3 x 1/3 = 2/9 hh 1/3 x 1/3= 1/9 tt 2/3 x 2/3 = 4/9 Answer: 5/9 = 0.56 (sum of weights in red) LSA 352 Summer 2007 27 Moving toward language What’s the probability of drawing a 2 from a deck of 52 cards with four 2s? 4 1 P(drawing a two) .077 52 13 What’s the probability of a random word (from a random dictionary page) being a verb? #of ways to get a verb P(drawing a verb) all words LSA 352 Summer 2007 28 Probability and part of speech tags • What’s the probability of a random word (from a random dictionary page) being a verb? P(drawing a verb) #of ways to get a verb all words • How to compute each of these • All words = just count all the words in the dictionary • # of ways to get a verb: number of words which are verbs! • If a dictionary has 50,000 entries, and 10,000 are verbs…. P(V) is 10000/50000 = 1/5 = .20 LSA 352 Summer 2007 29 Conditional Probability A way to reason about the outcome of an experiment based on partial information In a word guessing game the first letter for the word is a “t”. What is the likelihood that the second letter is an “h”? How likely is it that a person has a disease given that a medical test was negative? A spot shows up on a radar screen. How likely is it that it corresponds to an aircraft? LSA 352 Summer 2007 30 More precisely Given an experiment, a corresponding sample space S, and a probability law Suppose we know that the outcome is within some given event B We want to quantify the likelihood that the outcome also belongs to some other given event A. We need a new probability law that gives us the conditional probability of A given B P(A|B) LSA 352 Summer 2007 31 An intuition • A is “it’s raining now”. • P(A) in dry California is .01 • B is “it was raining ten minutes ago” • P(A|B) means “what is the probability of it raining now if it was raining 10 minutes ago” • P(A|B) is probably way higher than P(A) • Perhaps P(A|B) is .10 • Intuition: The knowledge about B should change our estimate of the probability of A. LSA 352 Summer 2007 32 Conditional probability One of the following 30 items is chosen at random What is P(X), the probability that it is an X? What is P(X|red), the probability that it is an X given that it is red? LSA 352 Summer 2007 33 Conditional Probability let A and B be events p(B|A) = the probability of event B occurring given event A occurs definition: p(B|A) = p(A B) / p(A) S QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. LSA 352 Summer 2007 34 Conditional probability P(A|B) = P(A B)/P(B) Or P( A | B) P( A, B) P( B) Note: P(A,B)=P(A|B) · P(B) Also: P(A,B) = P(B,A) A A,B B LSA 352 Summer 2007 35 Independence What is P(A,B) if A and B are independent? P(A,B)=P(A) · P(B) iff A,B independent. P(heads,tails) = P(heads) · P(tails) = .5 · .5 = .25 Note: P(A|B)=P(A) iff A,B independent Also: P(B|A)=P(B) iff A,B independent LSA 352 Summer 2007 36 Bayes Theorem P( A | B) P( B) P( B | A) P( A) • Swap the conditioning • Sometimes easier to estimate one kind of dependence than the other LSA 352 Summer 2007 37 Deriving Bayes Rule P(A B) P(B | A) P(A B) P(A | B) P(A) P(B) P(A | B)P(B) P(A B) P(B | A)P(A) P(A B) | B)P(B) P(B | A)P(A) P(A P(B | A)P(A) P(A | B) P(B) LSA 352 Summer 2007 38 Summary Probability Conditional Probability Independence Bayes Rule LSA 352 Summer 2007 39 How many words? I do uh main- mainly business data processing Fragments Filled pauses Are cat and cats the same word? Some terminology Lemma: a set of lexical forms having the same stem, major part of speech, and rough word sense – Cat and cats = same lemma Wordform: the full inflected surface form. – Cat and cats = different wordforms LSA 352 Summer 2007 40 How many words? they picnicked by the pool then lay back on the grass and looked at the stars 16 tokens 14 types SWBD: ~20,000 wordform types, 2.4 million wordform tokens Brown et al (1992) large corpus 583 million wordform tokens 293,181 wordform types Let N = number of tokens, V = vocabulary = number of types General wisdom: V > O(sqrt(N)) LSA 352 Summer 2007 41 Language Modeling We want to compute P(w1,w2,w3,w4,w5…wn), the probability of a sequence Alternatively we want to compute P(w5|w1,w2,w3,w4,w5): the probability of a word given some previous words The model that computes P(W) or P(wn|w1,w2…wn1) is called the language model. A better term for this would be “The Grammar” But “Language model” or LM is standard LSA 352 Summer 2007 42 Computing P(W) How to compute this joint probability: P(“the”,”other”,”day”,”I”,”was”,”walking”,”along”,”and”,” saw”,”a”,”lizard”) Intuition: let’s rely on the Chain Rule of Probability LSA 352 Summer 2007 43 The Chain Rule of Probability Recall the definition of conditional probabilities Rewriting: P( A^ B) P( A | B) P( B) P( A^ B) P( A | B) P( B) More generally P(A,B,C,D) = P(A)P(B|A)P(C|A,B)P(D|A,B,C) In general P(x1,x2,x3,…xn) = P(x1)P(x2|x1)P(x3|x1,x2)…P(xn|x1…xn-1) LSA 352 Summer 2007 44 The Chain Rule Applied to joint probability of words in sentence P(“the big red dog was”)= P(the)*P(big|the)*P(red|the big)*P(dog|the big red)*P(was|the big red dog) LSA 352 Summer 2007 45 Very easy estimate: How to estimate? P(the|its water is so transparent that) P(the|its water is so transparent that) = C(its water is so transparent that the) _______________________________ C(its water is so transparent that) LSA 352 Summer 2007 46 Unfortunately There are a lot of possible sentences We’ll never be able to get enough data to compute the statistics for those long prefixes P(lizard|the,other,day,I,was,walking,along,and,saw,a) Or P(the|its water is so transparent that) LSA 352 Summer 2007 47 Markov Assumption Make the simplifying assumption P(lizard|the,other,day,I,was,walking,along,and,saw,a) = P(lizard|a) Or maybe P(lizard|the,other,day,I,was,walking,along,and,saw,a) = P(lizard|saw,a) LSA 352 Summer 2007 48 Markov Assumption So for each component in the product replace with the approximation (assuming a prefix of N) n1 1 P(wn | w ) P(wn | w n1 nN 1 ) Bigram version n1 1 P(wn | w ) P(wn | wn1) LSA 352 Summer 2007 49 Estimating bigram probabilities The Maximum Likelihood Estimate count(w i1,w i ) P(w i | w i1) count(w i1 ) c(w i1,w i ) P(w i | w i1) c(w i1) LSA 352 Summer 2007 50 An example <s> I am Sam </s> <s> Sam I am </s> <s> I do not like green eggs and ham </s> This is the Maximum Likelihood Estimate, because it is the one which maximizes P(Training set|Model) LSA 352 Summer 2007 51 Maximum Likelihood Estimates The maximum likelihood estimate of some parameter of a model M from a training set T Is the estimate that maximizes the likelihood of the training set T given the model M Suppose the word Chinese occurs 400 times in a corpus of a million words (Brown corpus) What is the probability that a random word from some other text will be “Chinese” MLE estimate is 400/1000000 = .004 This may be a bad estimate for some other corpus But it is the estimate that makes it most likely that “Chinese” will occur 400 times in a million word corpus. LSA 352 Summer 2007 52 More examples: Berkeley Restaurant Project sentences can you tell me about any good cantonese restaurants close by mid priced thai food is what i’m looking for tell me about chez panisse can you give me a listing of the kinds of food that are available i’m looking for a good place to eat breakfast when is caffe venezia open during the day LSA 352 Summer 2007 53 Raw bigram counts Out of 9222 sentences LSA 352 Summer 2007 54 Raw bigram probabilities Normalize by unigrams: Result: LSA 352 Summer 2007 55 Bigram estimates of sentence probabilities P(<s> I want english food </s>) = p(i|<s>) x p(want|I) x p(english|want) x p(food|english) x p(</s>|food) = .24 x .33 x .0011 x 0.5 x 0.68 =.000031 LSA 352 Summer 2007 56 What kinds of knowledge? P(english|want) = .0011 P(chinese|want) = .0065 P(to|want) = .66 P(eat | to) = .28 P(food | to) = 0 P(want | spend) = 0 P (i | <s>) = .25 LSA 352 Summer 2007 57 The Shannon Visualization Method Generate random sentences: Choose a random bigram <s>, w according to its probability Now choose a random bigram (w, x) according to its probability And so on until we choose </s> Then string the words together <s> I I want want to to eat eat Chinese Chinese food food </s> LSA 352 Summer 2007 58 LSA 352 Summer 2007 59 Shakespeare as corpus N=884,647 tokens, V=29,066 Shakespeare produced 300,000 bigram types out of V2= 844 million possible bigrams: so, 99.96% of the possible bigrams were never seen (have zero entries in the table) Quadrigrams worse: What's coming out looks like Shakespeare because it is Shakespeare LSA 352 Summer 2007 60 The wall street journal is not shakespeare (no offense) LSA 352 Summer 2007 61 Evaluation We train parameters of our model on a training set. How do we evaluate how well our model works? We look at the models performance on some new data This is what happens in the real world; we want to know how our model performs on data we haven’t seen So a test set. A dataset which is different than our training set Then we need an evaluation metric to tell us how well our model is doing on the test set. One such metric is perplexity (to be introduced below) LSA 352 Summer 2007 62 Unknown words: Open versus closed vocabulary tasks If we know all the words in advanced Vocabulary V is fixed Closed vocabulary task Often we don’t know this Out Of Vocabulary = OOV words Open vocabulary task Instead: create an unknown word token <UNK> Training of <UNK> probabilities – Create a fixed lexicon L of size V – At text normalization phase, any training word not in L changed to <UNK> – Now we train its probabilities like a normal word At decoding time – If text input: Use UNK probabilities for any word not in training LSA 352 Summer 2007 63 Evaluating N-gram models Best evaluation for an N-gram Put model A in a speech recognizer Run recognition, get word error rate (WER) for A Put model B in speech recognition, get word error rate for B Compare WER for A and B In-vivo evaluation LSA 352 Summer 2007 64 Difficulty of in-vivo evaluation of N-gram models In-vivo evaluation This is really time-consuming Can take days to run an experiment So As a temporary solution, in order to run experiments To evaluate N-grams we often use an approximation called perplexity But perplexity is a poor approximation unless the test data looks just like the training data So is generally only useful in pilot experiments (generally is not sufficient to publish) But is helpful to think about. LSA 352 Summer 2007 65 Perplexity Perplexity is the probability of the test set (assigned by the language model), normalized by the number of words: Chain rule: For bigrams: Minimizing perplexity is the same as maximizing probability The best language model is one that best predicts an unseen test set LSA 352 Summer 2007 66 A totally different perplexity Intuition How hard is the task of recognizing digits ‘0,1,2,3,4,5,6,7,8,9,oh’: easy, perplexity 11 (or if we ignore ‘oh’, perplexity 10) How hard is recognizing (30,000) names at Microsoft. Hard: perplexity = 30,000 If a system has to recognize Operator (1 in 4) Sales (1 in 4) Technical Support (1 in 4) 30,000 names (1 in 120,000 each) Perplexity is 54 Perplexity is weighted equivalent branching factor Slide from Josh Goodman LSA 352 Summer 2007 67 Perplexity as branching factor LSA 352 Summer 2007 68 Lower perplexity = better model Training 38 million words, test 1.5 million words, WSJ LSA 352 Summer 2007 69 Lesson 1: the perils of overfitting N-grams only work well for word prediction if the test corpus looks like the training corpus In real life, it often doesn’t We need to train robust models, adapt to test set, etc LSA 352 Summer 2007 70 Lesson 2: zeros or not? Zipf’s Law: A small number of events occur with high frequency A large number of events occur with low frequency You can quickly collect statistics on the high frequency events You might have to wait an arbitrarily long time to get valid statistics on low frequency events Result: Our estimates are sparse! no counts at all for the vast bulk of things we want to estimate! Some of the zeroes in the table are really zeros But others are simply low frequency events you haven't seen yet. After all, ANYTHING CAN HAPPEN! How to address? Answer: Estimate the likelihood of unseen N-grams! Slide adapted from Bonnie Dorr and Julia Hirschberg LSA 352 Summer 2007 71 Smoothing is like Robin Hood: Steal from the rich and give to the poor (in probability mass) Slide from Dan Klein LSA 352 Summer 2007 72 Laplace smoothing Also called add-one smoothing Just add one to all the counts! Very simple MLE estimate: Laplace estimate: Reconstructed counts: LSA 352 Summer 2007 73 Laplace smoothed bigram counts LSA 352 Summer 2007 74 Laplace-smoothed bigrams LSA 352 Summer 2007 75 Reconstituted counts LSA 352 Summer 2007 76 Note big change to counts C(count to) went from 608 to 238! P(to|want) from .66 to .26! Discount d= c*/c d for “chinese food” =.10!!! A 10x reduction So in general, Laplace is a blunt instrument Could use more fine-grained method (add-k) But Laplace smoothing not used for N-grams, as we have much better methods Despite its flaws Laplace (add-k) is however still used to smooth other probabilistic models in NLP, especially For pilot studies in domains where the number of zeros isn’t so huge. LSA 352 Summer 2007 77 Better discounting algorithms Intuition used by many smoothing algorithms Good-Turing Kneser-Ney Witten-Bell Is to use the count of things we’ve seen once to help estimate the count of things we’ve never seen LSA 352 Summer 2007 78 Good-Turing: Josh Goodman intuition Imagine you are fishing There are 8 species: carp, perch, whitefish, trout, salmon, eel, catfish, bass You have caught 10 carp, 3 perch, 2 whitefish, 1 trout, 1 salmon, 1 eel = 18 fish How likely is it that next species is new (i.e. catfish or bass) 3/18 Assuming so, how likely is it that next species is trout? Must be less than 1/18 Slide adapted from Josh Goodman LSA 352 Summer 2007 79 Good-Turing Intuition Notation: Nx is the frequency-of-frequency-x So N10=1, N1=3, etc To estimate total number of unseen species Use number of species (words) we’ve seen once c0* =c1 p0 = N1/N All other estimates are adjusted (down) to give probabilities for unseen Slide from Josh Goodman LSA 352 Summer 2007 80 Good-Turing Intuition Notation: Nx is the frequency-of-frequency-x So N10=1, N1=3, etc To estimate total number of unseen species Use number of species (words) we’ve seen once c0* =c1 p0 = N1/N p0=N1/N=3/18 All other estimates are adjusted (down) to give probabilities for unseen P(eel) = c*(1) = (1+1) 1/ 3 = 2/3 Slide from Josh Goodman LSA 352 Summer 2007 81 LSA 352 Summer 2007 82 Bigram frequencies of frequencies and GT re-estimates LSA 352 Summer 2007 83 Complications In practice, assume large counts (c>k for some k) are reliable: That complicates c*, making it: Also: we assume singleton counts c=1 are unreliable, so treat N-grams with count of 1 as if they were count=0 Also, need the Nk to be non-zero, so we need to smooth (interpolate) the Nk counts before computing c* from them LSA 352 Summer 2007 84 Backoff and Interpolation Another really useful source of knowledge If we are estimating: trigram p(z|xy) but c(xyz) is zero Use info from: Bigram p(z|y) Or even: Unigram p(z) How to combine the trigram/bigram/unigram info? LSA 352 Summer 2007 85 Backoff versus interpolation Backoff: use trigram if you have it, otherwise bigram, otherwise unigram Interpolation: mix all three LSA 352 Summer 2007 86 Interpolation Simple interpolation Lambdas conditional on context: LSA 352 Summer 2007 87 How to set the lambdas? Use a held-out corpus Choose lambdas which maximize the probability of some held-out data I.e. fix the N-gram probabilities Then search for lambda values That when plugged into previous equation Give largest probability for held-out set Can use EM to do this search LSA 352 Summer 2007 88 Katz Backoff LSA 352 Summer 2007 89 Why discounts P* and alpha? MLE probabilities sum to 1 So if we used MLE probabilities but backed off to lower order model when MLE prob is zero We would be adding extra probability mass And total probability would be greater than 1 LSA 352 Summer 2007 90 GT smoothed bigram probs LSA 352 Summer 2007 91 Intuition of backoff+discounting How much probability to assign to all the zero trigrams? Use GT or other discounting algorithm to tell us How to divide that probability mass among different contexts? Use the N-1 gram estimates to tell us What do we do for the unigram words not seen in training? Out Of Vocabulary = OOV words LSA 352 Summer 2007 92 OOV words: <UNK> word Out Of Vocabulary = OOV words We don’t use GT smoothing for these Because GT assumes we know the number of unseen events Instead: create an unknown word token <UNK> Training of <UNK> probabilities – Create a fixed lexicon L of size V – At text normalization phase, any training word not in L changed to <UNK> – Now we train its probabilities like a normal word At decoding time – If text input: Use UNK probabilities for any word not in training LSA 352 Summer 2007 93 Practical Issues We do everything in log space Avoid underflow (also adding is faster than multiplying) LSA 352 Summer 2007 94 ARPA format LSA 352 Summer 2007 95 LSA 352 Summer 2007 96 Language Modeling Toolkits SRILM CMU-Cambridge LM Toolkit LSA 352 Summer 2007 97 Google N-Gram Release LSA 352 Summer 2007 98 Google N-Gram Release serve serve serve serve serve serve serve serve serve serve as as as as as as as as as as the the the the the the the the the the incoming 92 incubator 99 independent 794 index 223 indication 72 indicator 120 indicators 45 indispensable 111 indispensible 40 individual 234 LSA 352 Summer 2007 99 Advanced LM stuff Current best smoothing algorithm Kneser-Ney smoothing Other stuff Variable-length n-grams Class-based n-grams – Clustering – Hand-built classes Cache LMs Topic-based LMs Sentence mixture models Skipping LMs Parser-based LMs LSA 352 Summer 2007 100 Summary LM N-grams Discounting: Good-Turing Katz backoff with Good-Turing discounting Interpolation Unknown words Evaluation: – Entropy, Entropy Rate, Cross Entropy – Perplexity Advanced LM algorithms LSA 352 Summer 2007 101