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Learning Language from Distributional Evidence Christopher Manning Depts of CS and Linguistics Stanford University Workshop on Where Does Syntax Come From?,

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Presentation on theme: "Learning Language from Distributional Evidence Christopher Manning Depts of CS and Linguistics Stanford University Workshop on Where Does Syntax Come From?,"— Presentation transcript:

1 Learning Language from Distributional Evidence Christopher Manning Depts of CS and Linguistics Stanford University Workshop on Where Does Syntax Come From?, MIT, Oct 2007

2 2 There’s a lot to agree on! [C. D. Yang UG, statistics or both? Trends in CogSci 8(10)] Both endowment (priors/biases) and learning from experience contribute to language acquisition “To be effective, a learning algorithm … must have an appropriate representation of the relevant … data.” Languages, and hence models of language, have intricate structure, which must be modeled.

3 3 More points of agreement Learning language structure requires priors or biases, to work at all, but especially at speed Yang is “in favor of probabilistic learning mechanisms that may well be domain-general” I am too. Probabilistic methods (and especially Bayesian prior + likelihood methods) are perfect for this! Probabilistic models can achieve and explain: gradual learning and robustness in acquisition non-homogeneous grammars of individuals gradual language change over time [and also other stuff, like online processing]

4 4 The disagreements are important, but two levels down In his discussion of Saffran, Aslin, and Newport (1998), Yang contrasts “statistical learning” (of syllable bigram transition probabilities) from use of UG principles such as one primary stress per word. I would place both parts in the same probabilistic model Stress is a cue for word learning, just as syllable transition probabilities are a cue (and it’s very subtle in speech!!!) Probability theory is an effective means of combining multiple, often noisy, sources of information with prior beliefs Yang keeps probabilities outside of the grammar, by suggesting that the child maintains a probability distribution over a collection of competing grammars I would place the probabilities inside the grammar It is more economical, explanatory, and effective.

5 5 The central questions 1. What representations are appropriate for human languages? 2. What biases are required to learn languages successfully? Linguistically informed biases – but perhaps fairly general ones are enough 3. How much of language structure can be acquired from the linguistic input? This gives a lower bound on how much is innate.

6 6 1. A mistaken meme: language as a homogeneous, discrete system Joos (1950: 701–702): “Ordinary mathematical techniques fall mostly into two classes, the continuous (e.g., the infinitesimal calculus) and the discrete or discontinuous (e.g., finite group theory). Now it will turn out that the mathematics called ‘linguistics’ belongs to the second class. It does not even make any compromise with continuity as statistics does, or infinite-group theory. Linguistics is a quantum mechanics in the most extreme sense. All continuities, all possibilities of infinitesimal gradation, are shoved outside of linguistics in one direction or the other.” [cf. Chambers 1995]

7 7 The quest for homogeneity Bloch (1948: 7): “The totality of the possible utterances of one speaker at one time in using a language to interact with one other speaker is an idiolect. … The phrase ‘with one other speaker’ is intended to exclude the possibility that an idiolect might embrace more than one style of speaking.” Sapir (1921: 147) “Everyone knows that language is variable”

8 8 Variation is everywhere The definition of an idiolect fails, as variation occurs even internal to the usage of a speaker in one style. As least as: Black voters also turned out at least as well as they did in 1996, if not better in some regions, including the South, according to exit polls. Gore was doing as least as well among black voters as President Clinton did that year. (Associated Press, 2000)

9 9 Linguistic Facts vs. Linguistic Theories Weinreich, Labov and Herzog (1968) see 20 th century linguistics as having gone astray by mistakenly searching for homogeneity in language, on the misguided assumption that only homogeneous systems can be structured Probability theory provides a method for describing structure in variable systems!

10 10 The need for probability models inside syntax The motivation comes from two sides: Categorical linguistic theories claim too much: They place a hard categorical boundary of grammaticality, where really there is a fuzzy edge, determined by many conflicting constraints and issues of conventionality vs. human creativity Categorical linguistic theories explain too little: They say nothing at all about the soft constraints which explain how people choose to say things Something that language educators, computational NLP people – and historical linguists and sociolinguists dealing with real language – usually want to know about

11 11 Clausal argument subcategorization frames Problem: in context, language is used more flexibly than categorical constraints suggest: E.g., most subcategorization frame ‘facts’ are wrong Pollard and Sag (1994) inter alia [regard vs. consider]: *We regard Kim to be an acceptable candidate We regard Kim as an acceptable candidate The New York Times: As 70 to 80 percent of the cost of blood tests, like prescriptions, is paid for by the state, neither physicians nor patients regard expense to be a consideration. Conservatives argue that the Bible regards homosexuality to be a sin. And the same pattern repeats for many verbs and frames….

12 12       Probability mass functions: subcategorization of regard

13 13 Bresnan and Nikitina (2003) on the Dative Alternation Pinker (1981), Krifka (2001): verbs of instantaneous force allow dative alternation but not “verbs of continuous imparting of force” like push “As Player A pushed him the chips, all hell broke loose at the table.” id=165 Pinker (1981), Levin (1993), Krifka (2001): verbs of instrument of communication allow dative shift but not verbs of manner of speaking “Hi baby.” Wade says as he stretches. You just mumble him an answer. You were comfy on that soft leather couch. Besides … In context such usages are unremarkable! It’s just the productivity and context-dependence of language Examples are rare → because these are gradient constraints. Here data is gathered from a really huge corpus: the web.

14 14 The disappearing hard constraints of categorical grammars We see the same thing over and over Another example is constraints on Heavy NP Shift [cf. Wasow 2002] You start with a strong categorical theory, which is mostly right. People point out exceptions and counter examples You either weaken it in the face of counterexamples Or you exclude the examples from consideration Either way you end up without an interesting theory There’s little point in aiming for explanatory adequacy when the descriptive adequacy of the used representations [as opposed to particular descriptions] just isn’t there. There is insight in the probability distributions!

15 15 Explaining more: What do people say? What people say has two parts: Contingent facts about the world People have been talking a lot about Iraq lately The way speakers choose to express ideas using the resources of the language People don’t often put that clauses pre-verbally: It appears almost certain that we will have to take a loss That we will have to take a loss appears almost certain The latter is properly part of people’s Knowledge of Language. Part of syntax.

16 16 Variation is part of competence [Labov 1972: 125] “The variable rules themselves require at so many points the recognition of grammatical categories, of distinctions between grammatical boundaries, and are so closely interwoven with basic categorical rules, that it is hard to see what would be gained by extracting a grain of performance from this complex system. It is evident that [both the categorical and the variable rules proposed] are a part of the speaker’s knowledge of language.”

17 17 What do people say? Simply delimiting a set of grammatical sentences provides only a very weak description of a language, and of the ways people choose to express ideas in it Probability densities over sentences and sentence structures can give a much richer view of language structure and use In particular, we find that (apparently) categorical constraints in one language often reappear as the same soft generalizations and tendencies in other languages [Givón 1979, Bresnan, Dingare, and Manning 2001] Linguistic theory should be able to uniformly capture these constraints, rather than only recognizing them when they are categorical

18 18 Explaining more: what determines ditransitive vs. NP PP for dative verb [Bresnan, Cueni, Nikitina, and Baayen 2005] Build mixed effects [logistic regression] model over a corpus of examples Model is able to pull apart the correlations between various predictive variables Explanatory variables: Discourse accessibility, definiteness, pronominality, animacy (Thompson 1990, Collins 1995) Differential length in words of recipient and theme (Arnold et al. 2000, Wasow 2002, Szmrecsanyi 2004b) Structural parallelism in dialogue (Weiner and Labov 1983, Bock1986, Szmrecsanyi 2004a) Number, person (Aissen1999, 2003; Haspelmath 2004; Bresnan and Nikitina 2003) Concreteness of theme Broad semantic class of verb (transfer, prevent, communicate, …)

19 19 Explaining more: what determines ditransitive vs. NP PP for dative verb [Bresnan, Cueni, Nikitina, and Baayen 2005] What does one learn? Almost all the predictive variables have independent significant effects Only a couple fail to: e.g., number of recipient Shows that reductionist theories of the phenomenon that tries to reduce things to one or two factors are wrong First object NP is preferred to be: Given, animate, definite, pronoun, shorter Model can predict whether to use a double object or NP PP construction correctly 94% of the time. It captures much of what is going on in this choice These factors exceed in importance differences from individual variation

20 20 the reporter who the senator attacked Statistical parsing models also give insight into processing [Levy 2004] A realistic model of human sentence processing explain: Robustness to arbitrary input, accurate disambiguation Inference on the basis of incomplete input [Tanenhaus et al 1995, Altmann and Kamide 1999, Kaiser and Trueswell 2004] Sentence processing difficulty is differential and localized On the traditional view, resource limitations, especially memory, drive processing difficulty Locality-driven processing [Gibson 1998, 2000]: multiple and/or more distant dependencies are harder to process the reporter who attacked the senator Processing Easy Hard

21 21 Expectation-driven processing [Hale 2001, Levy 2005] Alternative paradigm: Expectation-based models of syntactic processing Expectations are weighted averages over probabilities Structures we expect are easy to process Modern computational linguistics techniques of statistical parsing  precise psycholinguistic model Model matches empirical results of many recent experiments better than traditional memory-limitation models

22 22 Example: Verb-final domains Locality predictions and empirical results [Konieczny 2000] looked at reading times at German final verbs Locality-based models (Gibson 1998) predict difficulty for longer clauses But Konieczny found that final verbs were read faster in longer clauses Prediction easy hard Result fast fastest slow Er hat die Gruppe auf den Berg geführt Er hat die Gruppe geführt He led the group He led the group to the mountain Er hat die Gruppe auf den sehr schönen Berg geführt He led the group to the very beautiful mountain

23 23 Once we’ve seen a PP goal we’re unlikely to see another So the expectation of seeing anything else goes up Rigorously tested: for p i (w), using a PCFG derived empirically from a syntactically annotated corpus of German (the NEGRA treebank) Seeing more = having more information More information = more accurate expectations Deriving Konieczny’s results auf den Berg PP geführt V NP? PP-goal? PP-loc? Verb? ADVP? die Gruppe VP NP S Vfin Er hat

24 24 Er hat die Gruppe (auf den (sehr schönen) Berg) geführt Predictions from the Hale 2001/Levy 2004 model Locality-based models (e.g., Gibson 1998, 2000) would violate monotonicity Locality-based difficulty (ordinal) 1 2 3

25 25 2.&3. Learning sentence structure from distributional evidence Start with raw language, learn syntactic structure Some have argued that learning syntax from positive data alone is impossible: Gold, 1967: Non-identifiability in the limit Chomsky, 1980 : The poverty of the stimulus Many others have felt it should be possible: Lari and Young, 1990 Carroll and Charniak, 1992 Alex Clark, 2001 Mark Paskin, 2001 … but it is a hard problem

26 26 Language learning idea 1: Lexical affinity models Words select other words on syntactic grounds Link up pairs with high mutual information [Yuret, 1998]: Greedy linkage [Paskin, 2001]: Iterative re-estimation with EM Evaluation: compare linked pairs (undirected) to a gold standard congress narrowly passed the amended bill 39.7 AccuracyMethod Paskin, Random

27 27 Idea: Word Classes Mutual information between words does not necessarily indicate syntactic selection. Individual words like brushbacks are entwined with semantic facts about the world. Syntactic classes, like NOUN and ADVERB are bleached of word-specific semantics. We could build dependency models over word classes. [cf. Carroll and Charniak, 1992] NOUN ADVERB VERB DET PARTICIPLE NOUN congress narrowly passed the amended bill expect brushbacks but no beanballs

28 28 Problems: Word Class Models Issues: Too simple a model – doesn’t work much better supervised No representation of valence/distance of arguments) NOUN NOUN VERB stock prices fell NOUN NOUN VERB stock prices fell 41.7Random Adjacent Words Carroll and Charniak, 92 congress narrowly passed the amended bill

29 29 Bias: Using better dependency representations [Klein and Manning 2004] Classes?DistanceLocal Factor Paskin 01 P(a | h) Carroll & Charniak 92 P(c(a) | c(h)) Klein/Manning (DMV) P(c(a) | c(h), d) arghead distance ? 55.9Adjacent Words 63.6Klein/Manning (DMV)

30 30 Idea: Can we learn phrase structure constituency as distributional clustering presidentthe __ of presidentthe __ said governorthe __ of governorthe __ appointed said sources __  saidpresident __ that reported sources __  president governor said reported the a  the president said that the downturn was over  [Finch and Chater 92, Schütze 93, many others]

31 31 Distributional learning There is much debate in the child language acquisition literature about distributional learning and the possibility of kids successfully using it: [Maratsos & Chalkley 1980] suggest it [Pinker 1984] suggests it‘s impossible (too many correlations, too abstract properties needed) [Reddington et al. 1998] say it succeeds because there are dominant cues relevant to language [Mintz et al. 2002] look at distributional structure of input Speaking in terms of engineering, let me just tell you that it works really well! It’s one of the most successful techniques that we have – my group uses it everywhere for NLP.

32 32 Idea: Distributional Syntax? [Klein and Manning NIPS 2003]  factory payrolls fell in september  NP PP VP S payrolls __  fell in september ContextSpan factory __ septpayrolls fell in Can we use distributional clustering for learning syntax?

33 33 Problem: Identifying Constituents the final vote two decades most people decided to took most of go with of the with a without many in the end on time for now the final the intitial two of the Distributional classes are easy to find… PP VP NP - + … but figuring out which are constituents is hard. Principal Component 2 Principal Component 1

34 34 A Nested Distributional Model: Constituent-Context Model (CCM) P(S|T) =  factory payrolls fell in september  P( fpfis |+) P(  __  |+) P( fp |+) P(  __ fell |+) P( fis |+) P( p __  |+) P( is |+) P( fell __  |+) + + +

35 35 Initialization: A little UG? Tree Uniform Split Uniform

36 36 Results: Constituency van Zaanen, Right-Branch70.0 Our Model (CCM)81.6 Treebank Parse CCM Parse

37 37 Combining the two models [Klein and Manning ACL 2004] Supervised PCFG constituency recall is at 92.8 Qualitative improvements Subject-verb groups gone, modifier placement improved Random45.6 DMV62.7 CCM + DMV64.7 Random39.4 CCM81.0 CCM + DMV88.0 ! Constituency Evaluation Dependency Evaluation

38 38 Beyond surface syntax… [Levy and Manning, ACL 2004] origin? Syntactic category, parent, grandparent (subj vs obj extraction; VP finiteness Syntactic path (Gildea & Jurafsky 2002): Plus: feature conjunctions, specialized features for expletive subject dislocations, passivizations, passing featural information properly through coordinations, etc., etc. cf. Campbell (2004 ACL) – a lot of linguistic knowledge Presence of daughters (NP under S) Head words (wanted vs to vs eat)

39 39 Evaluation on dependency metric: gold-standard input trees

40 40 She opened the door with a key. Fortunately, the corkscrew opened the bottle. The bottle opened easily. The door opened as they approached. This key opens the door of the cottage. She opened the bottle very carefully. He opened the bottle with a corkscrew. He opened the door. Instances of open: instrument patient agent she, he door, bottle key, corkscrew Word Distributions: { agent=subject, patient=object } { patient=subject } { agent=subject, patient=object, instrument=obl_with } { instrument=subject, patient=object } Allowed Linkings: Might we also learn a linking to argument structure distributionally? Why it might be possible:

41 41 Probabilistic Model Learning [Grenager and Manning 2006 EMNLP] v v l l o o give { 0=subj, 1=obj2, 2=obj1 } [ 0=subj, M=np 1=obj2, 2=obj1 ] r1r1 r1r1 w1w1 w1w1 subj0plunge s1s1 s1s1 r2r2 r2r2 w2w2 w2w2 npMtoday s2s2 s2s2 r3r3 r3r3 w3w3 w3w3 obj12them s3s3 s3s3 r4r4 r4r4 s4s4 s4s4 w4w4 w4w4 obj21test Given a set of observed verb instances, what are the most likely model parameters? Use unsupervised learning in a structured probabilistic graphical model A good application for EM! M-step: Trivial computation E-Step: We compute conditional distributions over possible role vectors for each instance And we repeat

42 42 Verb: give Linkings: Roles: {0=subj,1=obj2, 2=obj1} 0.46 {0=subj,1=obj1, 2=to} {0=subj,1=obj1} 0.19 … … … … Verb: pay Linkings: Roles: {0=subj,1=obj1} 0.32 {0=subj,1=obj1, 2=for} {0=subj} 0.21 {0=subj,1=obj1, 2=to} … … {0=subj,1=obj2, 2=obj1} … … it, he, bill, they, that, … power, right, stake, … them, it, him, dept., … … … 0.05 … … it, they, company, he, … $, bill, price, tax … stake, gov., share, amt., … … … … … Semantic role induction results The model achieve some traction, but it’s hard Learning becomes harder with greater abstraction This is the right research frontier to explore!

43 43 Conclusions Probabilistic models give precise descriptions of a variable, uncertain world There are many phenomena in syntax that cry out for non- categorical or probabilistic representations of language Probabilistic models can –and should – be used over rich linguistic representations They support effective learning and processing Language learning does require biases or priors But a lot more can be learned from modest amounts of input than people have thought There’s not much evidence of a poverty of the stimulus preventing such models being used in acquisition.


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