CS460/626 : Natural Language

Transcription

1 CS460/626 : Natural Language Processing/Speech, NLP and the Web (Lecture 27 SMT Assignment; HMM recap; Probabilistic Parsing cntd) Pushpak Bhattacharyya CSE Dept., IIT Bombay 17 th March, 2011

2 CMU Pronunciation Dictionary Assignment

3 Data The Carnegie Mellon University Pronouncing Dictionary machine-readable pronunciation dictionary for North American English that contains over 125,000 words and their transcriptions. The current phoneme set contains 39 phonemes

4 Parallel Corpus Phoneme Example Translation AA odd AA D AE at AE T AH hut HH AH T AO ought AO T AW cow K AW AY hide HH AY D B be B IY

5 Parallel Corpus cntd Phoneme Example Translation CH cheese CH IY Z D DH ER EY F G HH IH IY JH dee DIY thee DH IY EH Ed EH D hurt HH ER T ate EY T fee F IY green G R IY N he HH IY it IH T eat IY T gee JH IY

6 The tasks First obtain the Carnegie Mellon University's it Pronouncing Dictionary Create the Phrase Table using GIZA++ For language modeling use SRILM For decoding use Moses Calculate precision, recall and F-score

7 Probabilistic Parsing

8 Bridging Classical and Probabilistic Parsing The bridge between probabilistic parsing and classical parsing is the concept of domination Frequency: P( NP -> DT NN) = 0.5 means in the corpus 50% of noun phrase is composed of determiner and noun Phenomenon: P(NP -> DT NN) is actually P(DT NN NP) i.e. join probability of domination by DT and NN to give rise to domination of NP The concept of domination is the bridge between Frequency (probabilistic parsing) and Phenomenon (classical parsing).

9 Calculating Probability of a Sentence = 1 m = t: yield ( t ) = w P ( s w ) P ( t ) We can either calculate P(s=w 1 m ) using naive N-gram based approach or by calculating P() t t : yield ( t ) = w1 m Which approach to choose?? 1 m The velocity of waves rises near the shore. Consecutive plural noun and singular verb is unlikely in the corpus. So low probability value for the sentence as given by n- gram.

10 Parse Tree S NP VP NP PP V PP DT NN P NNS rises P near P NP N The velocity of waves the shore No other Parse Tree is possible for the sentence

11 Various ways to calculate probability of sentence Naïve n-gram based Syntactic level (Parse tree) Semantic Level Pragmatics Discourse

12 Probabilistic Context Free Grammars S NP VP DT the NP DT NN 0.5 NN gunman 0.5 NP NNS NN building 0.5 NP NP PP 0.2 VBD sprayed 1.0 PP PNP 1.0 NNS bullets 1.0 VP VP PP 0.6 VP VBD NP 0.4

13 Example Parse t 1` The gunman sprayed the building with bullets. S 1.0 NP 0.5 VP 0.6 P (t 1 ) = 1.0 * 0.5 * 1.0 * 0.5 * 0.6 * 0.4 * 1.0 * 0.5 * 1.0 * 0.5 * 1.0 * 1.0 * 0.3 * 1.0 = DT 1.0 NN VP 0.4 PP 1.0 The gunman VBD 1.0 NP 0.5 P 1.0 NP 0.3 sprayed DT 1.0 NN 0.5 with NNS 1.0 the building bullets

14 Another Parse t 2 The gunman sprayed the building with bullets. S 1.0 P (t 2 ) = 1.0 * 0.5 * 1.0 * 0.5 * NP 0.5 VP 0.4 DT 1.0 NN 0.5 VBD 1.0 NP * 1.0 * 0.2 * 0.5 * 1.0 * 0.5 * 1.0 * 1.0 * 0.3 * 1.0 = The gunman sprayed NP 0.5 PP 1.0 DT 1.0 NN 0.5 P 1.0 NP 0.3 th building with NNS 1.0 e bullet s

15 Probability of a parse tree (cont.) S 1,l NP 1,2 VP 3,l P ( t s ) = P (t S 1,l ) DT 1 N 2 V 3,3 PP 4,l = P ( NP 1,2, DT 1,1, w 1, w w P NP 1 2 w 3 4,4 5,l N 2,2, w 2, VP 3,l, V 3,3, w 3, PP 4,l, P 4,4, w 4, NP 5,l, w 5 l S 1,l ) w 4 w 5 w l = P ( NP 1,2, VP 3,l S 1,l ) * P ( DT 1,1, N 2,2 NP 1,2 ) * D(w 1 DT 1,1 ) * P( (w 2 N 2,2 )*P(V 3,3, PP 4,l VP 3,l )*P( P(w 3 V 3,3 )*P(P P 4,4, NP 5,l PP 4,l ) * P(w 4 P 4,4 ) * P (w 5 l NP 5,l ) (Using Chain Rule, Context Freeness and Ancestor Freeness )

16 HMM PCFG O observed sequence w 1m sentence X state sequence t parse tree μ model G grammar Three fundamental questions

17 HMM PCFG How likely is a certain observation given the model? How likely l is a sentence given the grammar? PO ( μ ) Pw ( G ) How to choose a state sequence which best explains the observations? How to choose a parse which best supports the sentence? 1m arg max PX ( Oμ, ) arg max P( t w1 m, G) X t

18 HMM PCFG How to choose the model parameters that best explain the observed data? How to choose rule probabilities which maximize the probabilities of the observed sentences? arg max PO ( ) μ μ P w1 m G arg max ( G)

19 Recap of HMM

20 HMM Definition Set of states: S where S =N Start state S 0 /*P(S 0 )=1*/ Output Alphabet: O where O =M Transition Probabilities: A= {a ij } /*state i to state j*/ Emission Probabilities : B= {b j (o k )} /*prob. of emitting or absorbing o k from state j*/ Initial State Probabilities: Π={p 1,p 2,p 3, p N } Each p i =P(o 0 =ε,s i S 0 )

21 Markov Processes Properties Limited Horizon: Given previous t states, a state i, is independent of preceding 0 to t- k+1 states. P(X t =i X t-1, X t-2, X 0 ) = P(X t =i X t-1, X t-2 X t-k ) Order k Markov process Time invariance: (shown for k=1) P(X t =i X t-1 =j) = P(X 1 =i X 0 =j) = P(X n =i X n-1 =j)

22 Three basic problems (contd.) Problem 1: Likelihood of a sequence Forward Procedure Backward Procedure Problem 2: Best state sequence Viterbi Algorithm Problem 3: Re-estimation Baum-Welch ( Forward-Backward Algorithm )

23 Probabilistic Inference O: Observation Sequence S: State Sequence Given O find S * * where S = arg max p( S / O) called Probabilistic Inference S Infer Hidden from Observed How is this inference different from logical inference based on propositional or predicate calculus?

24 Essentials of Hidden Markov Model 1. Markov + Naive Bayes 2. Uses both transition and observation probability O p ( S k S ) = p ( O / S ) p ( S / S ) k k + 1 k k k + 1 k 3. Effectively makes Hidden Markov Model a Finite State Machine (FSM) with probability

25 Probability of Observation Sequence po ( ) = pos (, ) S = ps ( ) po ( / S) S Without any restriction, Search space size= S O

26 Continuing with the Urn example Colored Ball choosing Urn 1 # of Red = 30 # of Green = 50 # of Blue = 20 Urn 2 # of Red = 10 # of Green = 40 # of Blue = 50 Urn 3 # of Red =60 # of Green =10 # of Blue = 30

27 Example (contd.) Given : Transition Probability U 1 U 2 U 3 U U U Observation/output Probability R G B U and U U Observation : RRGGBRGR What is the corresponding state sequence?

28 Diagrammatic representation (1/2) R, 0.3 G, 0.5 B, U U 3 R, B, 0.3 G, 0.1 R, 0.1 G, 0.4 B, 0.5 U 2 0.2

29 Diagrammatic representation (2/2) R,0.03 G,0.05 B,0.02 R,0.18 G,0.03 B,0.09 R,0.15 G,0.25 B,0.10 U 1 U 3 R, 0.08 G, 0.20 B, 0.12 R,0.06 G,0.24 B,0.30 U 2 R002 R,0.02 G,0.08 B,0.10 R,0.24 G004 G,0.04 B,0.12 R,0.18 G,0.03 B,0.09 R,0.02, G,0.08 B,0.10

30 Probabilistic FSM (a 1 :0.3) (a 1 :0.1) (a 2 :0.4) (a 1 :0.3) S 1 S 2 (a 2 :0.2) (a 1 :0.2) (a 2 :0.2) (a 2 :0.3) The question here is: what is the most likely state sequence given the output sequence seen

31 Developing the tree Start S1 S *0.1=0.1 S1 0.3 S2 0.0 S1 S2 0.0 a S1 S2 S1 S2 a 2 0.1*0.2= *0.4= *0.3= *0.2=0.06 Choose the winning sequence per state per iteration

32 Tree structure contd S1 S *0.1=0.009 S S S1 S a a 2 S1. S2 S1 S The problem bi being addressed d by this tree is S * = arg max P ( S a 1 a 2 a 1 a 2, μ ) a1-a2-a1-a2 is the output sequence and μ the model or the machine s

33 Viterbi Algorithm for the Urn problem (first two symbols) S 0 ε U 1 U 2 U 3 R U 1 U 2 U 3 U 1 U 2 U 3 U 1 U 2 U * * *: winner sequences

34 Markov process of order>1 (say 2) O 0 O 1 O 2 O 3 O 4 O 5 O 6 O 7 O 8 Obs: ε R R G G B R G R State: S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 Same theory works P(S).P(O S) P(O S) = P(O 0 S 0 ).P(S 1 S 0 ). [P(O 1 S 1 ). P(S 2 S 1 S 0 )]. [P(O 2 S 2 ). P(S 3 S 2 S 1 )]. [P(O 3 S 3 ).P(S 4 S 3 S 2 )]. [P(O 4 S 4 ).P(S 5 S 4 S 3 )]. [P(O 5 S 5 ).P(S 6 S 5 S 4 )]. [P(O 6 S 6 ).P(S 7 S 6 S 5 )]. [P(O 7 S 7 )P(S S ).P(S 8 7 S 6 )]. [P(O 8 S 8 ).P(S 9 S 8 S 7 )]. We introduce the states S 0 and ds 9 as initial i i and final states respectively. After S 8 the next state is S 9 with probability 1, i.e., P(S 9 S 8 S 7 )=1 O 0 is ε-transition

35 Adjustments Transition probability table will have tuples on rows and states on columns Output probability table will remain the same In the Viterbi tree, the Markov process will take effect from the 3 rd input symbol (εrr) There will be 27 leaves, out of which only 9 will remain Sequences ending in same tuples will be compared Instead of U1, U2 and U3 U 1 U 1, U 1 U 2, U 1 U 3, U 2 U 1, U 2 U 2,U 2 U 3, U 3 U 1,U 3 U 2,U 3 U 3

36 Forward and Backward Probability Calculation

37 Forward probability F(k,i) Define F(k,i)= Probability of being in t t S h i state S i having seen o 0 o 1 o 2 o k F(k,i)=P(o 0 o 1 o 2 o k, S i ) With m as the length of the observed sequence P(observed sequence)=p(o 0 o 1 o 2..o m ) =Σ p=0,n P(o 0 o 1 o 2..o m, S p ) =Σ p=0,n F(m, p)

38 Forward probability (contd.) F(k, q) = P(o 0 o 1 o 2..o k, S q ) = P(o 0 o 1 o 2..o k, S q ) = P(o 0 o 1 o 2..o k-1, o k,s q ) = Σ p=0,n P(o 0 o 1 o 2..o k-1, S p, o k,s q ) = Σ p=0,n P(o 0 o 1 o 2..o k-1, S p ). P(o m,s q o 0 o 1 o 2..o k-1, S p ) = Σ p=0,n F(k-1,p). P(o k,s q S p ) o k = Σ p=0,n F(k-1,p). P(S p S q ) O 0 O 1 O 2 O 3 O k O k+1 O m-1 O m k k+1 m-1 m S 0 S 1 S 2 S 3 S p S q S m S final

39 Backward probability B(k,i) Define B(k,i)= Probability of seeing o k o k+1 o k+2 o m given that t the state t was S i B(k,i)=P(o k o k+1 o k+2 o m \ S i ) With m as the length of the observed sequence P(observed sequence)=p(o 0 o 1 o 2..o m ) = P(o 0 o 1 o 2..o m S 0 ) =B(0,0) 0)

40 B(k, p) Backward probability (contd.) = P(o k o k+1 o k+2 o m \S p ) =P(o k+1 o k+2 o m, o k S p ) = Σ q=0,n P(o k+1 o k+2 o m, o k, S q S p ) = Σ q=0,n P(o k,s q S p ) P(o k+1 o k+2 o m o k,s q,s p ) = Σ q=0,n P(o k+1 o k+2 o m S q ). P(o k, S q S p ) o k = Σ q=0,n B(k+1,q). P(S p S q ) O 0 O 1 O 2 O 3 O k O k+1 O m-1 O m S 0 S 1 S 2 S 3 S p S q S m S final

41 Back to PCFG

42 Interesting Probabilities N 1 What is the probability of having a NP at this position such that it will derive the building? - β NP (4,5) NP Inside Probabilities The gunman sprayed the building with bullets Outside Probabilities biliti What is the probability of starting from N 1 and deriving The gunman sprayed, a NP and with bullets? - α NP (4,5)

43 Interesting Probabilities Random variables to be considered The non-terminal being expanded. E.g., NP The word-span covered by the non-terminal. E.g., (4,5) refers to words the building While calculating gp probabilities, consider: The rule to be used for expansion : E.g., NP DT NN The probabilities associated with the RHS nonterminals : E.g., DT subtree s inside/outside probabilities & NN subtree s inside/outside probabilities

44 Outside Probability α j(p,q) : The probability of beginning with N 1 & generating the non-terminal N j pq and all words outside w p..w q ( p, q) P( w, N, w G) j α j = 1( p 1) pq ( q+ 1) m N 1 N j w 1 w p-1 w p w q w q+1 w m

45 Inside Probabilities β j (p,q) : The probability of generating the words w p..w q starting with the non-terminal N j pq. β ( pq, ) = Pw ( N j, G) j pq pq N 1 α N j β w 1 w p-1 w p w q w q+1 w m

46 Outside & Inside Probabilities: example α = NP (4,5) for "the building" P (The gunman sprayed, NP,with bullets G) β (4,5) for "the building" = (the building, ) NP P NP4,5 G N 1 4,5 NP The gunman sprayed the building with bullets

47 Calculating Inside probabilities β j (p,q) Base case: j j β ( kk, ) = Pw ( N, G ) = PN ( w G ) j k kk kk k Base case is used for rules which derive the words or terminals directly Eg E.g., Suppose N j = NN is being considered & NN building is one of the rules with probability 0.5 β (5,5) = (, ) NN Pbuilding NN5,5 G = PNN ( bildi building G ) = 0.5 5,5

48 Induction Step: Assuming Grammar in Chomsky Normal Form Induction step : β ( pq, ) = Pw ( N, G) N j j j pq pq q 1 j r s N r N s, = PN ( NN)* rs d= p βr ( pd, )* w p w d w d+1 w q β s ( d + 1, q) Consider different splits of the words - indicated by d E.g., the huge building Split here for d=2 d=3 Consider different non-terminals to be used in the rule: NP DT NN, NP DT NNS are available options Consider summation over all these.

49 The Bottom-Up Approach The idea of induction Consider the gunman NP Base cases : Apply unary rules DT the Prob = 1.0 NN gunman Prob = 0.5 DT 1.0 NN 0.5 The gunman Induction : Prob that a NP covers these 2 words = P (NP DT NN) * P (DT deriving the word the ) * P (NN deriving the word gunman ) = 0.5 * 1.0 * 0.5 = 0.25

50 Parse Triangle A parse triangle is constructed cted for calculating lating β j (p,q) Probability of a sentence using β j (p,q): P( w G) = P( N w G) = P( w N, G) = β (1, m) 1 1 1m 1m 1m 1m 1

51 Parse Triangle The gunman sprayed the building with bullets (1) (2) (3) (4) (5) (6) (7) 1 β DT = β = NN β VBD = 1.0 β = 1.0 DT β NN = 0.5 β P = 1.0 β NNS = Fill diagonals with β j ( kk, )

52 Parse Triangle The gunman sprayed the building with bullets (1) (2) (3) (4) (5) (6) (7) β = 1.0 DT β NP = 0.25 β NN = 0.5 β = VBD 4 β DT = β = Calculate l using induction formula β NP = P NP1,2 G (1,2) (the gunman, ) = PNP ( DTNN)* βdt (1,1) 1) * βnn (2, 2) = 0.5*1.0*0.5 = 0.25 NN β P = 1.0 β NNS = 1.0

53 Example Parse t 1 The gunman sprayed the building with bullets. S 1.0 NP 0.5 VP 0.6 DT 1.0 NN 0.5 VP 0.4 PP 1.0 Rule used here is VP VP PP The gunman VBD 1.0 NP 0.5 P 1.0 NP 0.3 DT 1.0 NN 0.5 with NNS sprayed 1.0 th e building bullet s

54 Another Parse t 2 The gunman sprayed the building with bullets. S 1.0 Rule used here is NP 0.5 VP 0.4 VP VBD NP DT 1.0 NN 0.5 VBD 1.0 NP 0.2 The gunmansprayed NP 0.5 PP 1.0 DT 1.0 NN 0.5 P 1.0 NP 0.3 th building with NNS e bullet s

55 Parse Triangle The (1) gunman (2) sprayed (3) the (4) building (5) with (6) bullets (7) 1 β DT = β NP = β S = β NN = β VBD = 1.0 β VP = 1.0 β VP = β DT = 1.0 β NP = 0.25 β NP = β NN = β P = β PP = β NNS = 1.0 β (3,7) = (sprayed the building with bullets, ) VP P VP37 3,7 G = PVP ( VPPP)* βvp (3,5)* βpp (6,7) + P ( VP VBD NP)* β VBD (3,3)* β NP (4,7) = 0.6*1.0* *1.0*0.015 = 0.186

56 Different Parses Consider Different splitting points : E.g., 5th and 3 rd position Using different rules for VP expansion : E.g., VP VP PP, VP VBD NP Different parses for the VP sprayed the building with bullets can be constructed this way.

57 Outside Probabilities α j (p,q) Base case: α (1, m) = 1 for start symbol 1 α j (1, m) = 0 for j 1 Inductive step for calculating α j ( pq, : ) N 1 N f pe α f ( pe, ) PN ( f NN j g ) N j pq N g (q+1) e β ( q + g 1, e ) w 1 w p-1 w p w q w q+1 w e w e+1 w m Summation over f, g & e

58 Probability of a Sentence Pw (, N G) = Pw ( N, G) = α ( pq, ) β ( pq, ) j 1m pq 1m pq j j j j Joint probability of a sentence w 1m and that there is a constituent spanning words w p to w q is given as: P (The gunman...bullets, N G) N 1 NP 45 4,5 = P(The gunman...bullets N, G) j = α NP j 4,5 (4,5) βnp (4,5) + α (4,5) β (4,5) +... VP VP The gunman sprayed the building with bullets