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Why the “Word Sense Disambiguation Problem” can't be solved, and what should

be done instead

Patrick HanksMasaryk University, Brno

Czech Republichanks@fi.muni.cz

U. of WolverhamptonMay 2, 2007

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Traditional WSD procedure

• Take a list of senses of each word from a source (typically WordNet or LDOCE)

• Stipulate “disambiguation criteria” for different word senses

• Applies the criteria to unseen texts• Results: some successes; many failures

unresolved ambiguities; cases where no criteria are satisfied; etc.

• Declare success.

Yorick Wilks

• A leading figure in Artificial Intelligence.• His theory of preference semantics has been

hugely influential (on me and Paul Procter among many others and - through us - on lexicography)

• Wilks rightly characterizes the Semantic Web as “the apotheosis of annotation” and asks, “But what are its semantics?”– However, his 2005 paper (with Nancy Ide), Making

Sense about Sense, gets things wrong.

What sort of inventory?

• “Contemporary automatic WSD assigns sense labels drawn from a pre-defined sense inventory to words in context. ... If dictionaries are not a good source of sense inventories useful in NLP, where do we turn?” -- Ide and Wilks 2005– Yes, but for mapping meaning onto use, you also need a source of

syntagmatic inventories (including info about collocations).– Only Cobuild says anything systematically about syntagmatics of

each word. – The WSD people have never tried to use Cobuild.– LDOCE has no information about collocations.– You can’t extract information that isn’t there.

Ide and Wilks on lexicographers

• “Whatever kind of lexicographer one is dealing with, ... their goal is and must be the explanation of meaning to one who does not know it.”

– Ide and Wilks again– One might as well say:

• “Whatever kind of computational linguist one is dealing with, ... their goal is and must be the translation of texts into a foreign language without human intervention.”

– One of many goals of lexicographers is to compile inventories. Ask a suitably trained lexicographer to compile a syntagmatic inventory and he/she will compile one.

Ide and Wilks on “successful” WSD

• “All successful WSD has operated at ... the homograph rather than the sense level ... (e.g. “crane” = bird or machine) ... basically those [distinctions] that can be found easily in parallel texts in different languages.”

– Ide and Wilks again

– You mean, like French grue, Czech jeráb?

Does MT really succeed at the homograph level?

• Consider:– Eng. crane --> Ger. Hebewerkzeug, Kranich,

Kran.• Google’s t-translate makes a horrible mess

of: – A crane had built its nest on the roof vs. – They used a crane to lift the goods.

• The words are ambiguous but the contexts are unambiguous.

Reformulation of the aim of WSD

• WSD should aim to disambiguate all uses of words that are not ambiguous in the contexts in which they are used. – There is a crane in my garden

• ambiguous.– A crane had built its nest on the roof and – They used a crane to lift the goods

• not ambiguous.

What do we need?

• We need a dictionary of contexts.• There isn’t one.

Tim Berners-Lee

• Another hugely influential figure• Inventor of the word-wide web• Co-author (with Hendler and Lassila) of an

article in Scientific American (2001) predicting “the semantic web”.

Semantic Web: the dream

• To enable computers to manipulate data meaningfully.

• “Most of the Web's content today is designed for humans to read, not for computer programs to manipulate meaningfully.”

– Berners-Lee et al., 2001

Why are people so excited about the Semantic Web idea?

• It offers “unchecked exponential growth” of “data and information that can be processed automatically”

– Berners-Lee et al., 2001

• Distributed, not centrally controlled– but with scientists as “guardians of truth”? -Wilks

• “... paradoxes and unanswerable questions are a price that must be paid to achieve versatility.”

– Berners-Lee et al., 2001

Semantic Web: the reality

• RDF (Resource Description Framework) handles only html-tagged entities and precisely defined items.

• In SW jargon “ontology” means a list of names, addresses, documents, and other tagged, defined entities.

• The SW does not engage with natural language.• PREDICTION: If it does, then in the current

state of NLP it will come unstuck.

Three senses of ‘ontology’

• An ordered theory of everything that exists.– Aristotle, philosophers

• A list of all the concept words in a language (usually one that is ordered hierarchically)

– WordNet, computational linguists

• A list of names, addresses, dates, and other entities

– Semantic Web people

Semantic Web as a librarian

• All efforts devoted to tagging and classifying documents.

• The SW currently has neither the time nor the skill needed to look inside the documents and read what they say.

• If the dream is to be fulfilled, then sooner or later the SW must engage with the vague, fuzzy phenomenon that is meaning in natural language. It must learn to process unstructured text.

Hypertext

• “The power of hypertext is that anything can link to anything.” – Berners-Lee et al., 2001

• Yes, but we need procedures for determining (automatically) what counts as a relevant link, e.g. – Firing a person is relevant to employment law.– Firing a gun is relevant to warfare and criminal law.

Precise definition does not help discover implicatures

• The meaning of the English noun second is vague: “a short unit of time” and “1/60 of a minute”. – Wait a second. – He looked at her for a second.

• It is also a very precisely defined technical term in certain scientific contexts, the basic SI unit of time: – “the duration of 9,192,631,770 cycles of radiation

corresponding to the transition between two hyperfine levels of the ground state of an atom of caesium 133.”

Being precise about vagueness

• Giving a precise definition to an ordinary word removes it from ordinary language.

• When it is given a precise, stipulative definition, an ordinary word becomes a technical term.

• “An adequate definition of a vague concept must aim not at precision but at vagueness; it must aim at precisely that level of vagueness which characterizes the concept itself.”

– Wierzbicka 1985

A New Resource

• The Pattern Dictionary of English Verbs and their Arguments– Based on detailed, painstaking corpus pattern

analysis (CPA)– Drawing on a new, lexically based theory of

language, the “theory of norms and exploitations” (TNE)

– Being built in Brno (CZ) and Brandeis (USA)

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CPA (Corpus Pattern Analysis)

1. Identify usage patterns for each word– Patterns include semantic types and lexical sets of

arguments (valencies)

• Associate a meaning (“implicature”) with each pattern (not with the word in isolation)

• Match occurrences of the target word in unseen texts to the nearest pattern (“norm”)

• If 2 matches are found, choose the most frequent• If no match is found, it is not normal usage -- it is

an exploitation of a norm (or a mistake).

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Dictionaries and Ontologies• “Patterns include semantic types” .... What are these? • Dictionaries don’t show semantic type structure.• Ontologies such as WordNet and the Brandeis

Semantic Ontology (BSO) show a hierarchical structures of semantic types, e.g. a gun, pistol, revolver, rifle, cannon, mortar, Kalashnikov, ... is a: weapon

artifact physical object

entity

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Brandeis Semantic Ontology

• A hierarchy of semantic concepts, with links to words at the appropriate level.

• Example (shortened and edited a bit): Name: gun Type: FirearmInheritance tree: TopType > Entity > Material Entity

> Artifact > Weapon > FirearmTelic: Attack with Weapon

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Ontological reasoningEXAMPLE:

If it’s a gun, it must be a weapon, an artefact, a physical object, and an entity, and it is used for attacking people and things. – Otherwise known as ‘semantic inheritance’– So far, so good. – How useful is ontological information as a

basis for verbal reasoning? – Not as useful as we would like.

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Semantics and Usage (1)• He was pointing a gun at me

-- is a Weapon and a Material Entity.

BUT 2. A child’s toy gun

-- is an Entertainment Artifact, not a Weapon

3. The fastest gun in the west-- is a Human < Animate Entity, not a Weapon

• “must be a weapon” on the previous slide is too strong; should be “is probably a weapon”

• probabilities can be measured, using corpus data• The normal semantics of terms are constantly

exploited to make new concepts (as in 2 and 3)

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Semantics and Usage (2)

• Knowing the exact place of a word in a semantic ontology is not enough

• To compute meaning, we need more info....• Another major source of semantic information

(potentially) is usage: – how words go together (normally | unusually | never)

• How do patterns of usage (syntagmatic) mesh with the information in an ontology?– See Church and Hanks (1989), Cobuild, etc.

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The Semantics of Norms• Dennis closed his eyes and fired the gun

– [[Human]] fires [[Firearm]]

• He fired a single round at the soldiers– [[Human]] fires [[Projectile]] {at [[PhysObj = Target]]}

• BOTH PATTERNS MEAN: [[Human]] causes [[Firearm]] to discharge [[Projectile]] towards [[Target]]

• Rumsfeld fires anyone who stands up to him.– [[Human 1 = Employer]] fires [[Human 2 = Employee]]

• MEANS: discharge from employment

– The semantic roles Employer and Employee are assigned by context -- they are not part of the type structure of the language.

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Complications and DistractionsMinor senses:• reading this new book fired me with fresh

enthusiasm to visit this town– [[Event]] fire [[Human]] {with [[Attitude = Good]]}

• Mr. Walker fired questions at me.– [[Human 1]] fire [[Speech Act]] {at [[Human 2]]}

Named inanimate entity: • I ... got back on Mabel and fired herher up.

– Mabel is [[Artifact]] (a motorbike, actually) – [[Human]] fire [[Artifact > Energy Production Device]]

{up}

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What do you do with a gun?Word Sketch Engine: freq. of gun: BNC 5,269; OEC 91,781

48.70 (7)-330-load

54.96 (4)-663-aim

44. 79 (11)

---

42.55 (14)

46.35 (8)

44.87 (10)

61.37 (1)

60.96 (2)

OECBNC

1504

249

98

434

974

1639

1132

OECBNC

20.38 (7) 70 hold

20.58 (6) 20 wave

25.86 (5) 11brandish

27.77 (4) 31jump

28.42 (3) 85carry

30.80 (2) 59point

45.39 (1)104fire

Salience (rank)Frequency of collocation

Collocate (verb with gun as object)

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Shimmering Lexical Sets (1)

• weapon: carry, surrender, possess, use, deploy, fire, acquire, conceal, seize, ...

____• gun: fire, carry, pointpoint, jump, brandish, wave, hold,

cock, spike, load, reload, ...• rifle: fire, carry, sling (over one’s shoulder), load,

reload, aim, drop, clean, ...• pistol: fire, load, level, hold, brandish, point, carry,

wave, ... • revolver: empty, draw, hold, carry, take, ...

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Shimmering Lexical Sets (2)

• spear: thrust, hoist, carry, throw, brandish• sword: wield, draw, cross, brandish, swing,

sheathe, carry, ...• dagger: sheathe, draw, plunge, hold• sabre: wield, rattle, draw• knife: brandish, plunge, twist, wield• bayonet: fix

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Shimmering Lexical Sets (3)

• missile: fire, deploy, launch

• bullet: bite, fire, spray, shoot, put

• shell: fire, lob; crack, ...

• round: fire, shoot; ...

• arrow: fire, shoot, aim; paint, follow

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Shimmering Lexical Sets (4)• fire: shot, gun, bullet, rocket, missile, salvo ...

[[Projectile]] or [[Firearm]] • carry: passenger, weight, bag, load, burden, tray,

weapon, gun, cargo ... [polysemous]• aim: kick, measure, programme, campaign, blow,

mischief, policy, rifle ... [polysemous]• point: finger, gun, way, camera, toe, pistol ...

[polysemous?]• brandish: knife, sword, gun, shotgun, razor, stick,

weapon, pistol ... [[Weapon]]• shoot: glance, bolt, Palestinian, rapid, policeman;

– shoot ... with: pistol, bow, bullet, gun

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Triangulation

• Meanings attach to patterns, not words.• A typical pattern consists of a verb and its

arguments (with semantic values), thus: [[Human]] fire [[Projectile]] {from [[Firearm]]} {PREP [[PhysObj]]}

• Pattern elements are often omitted in actual usage. (See FrameNet)

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Semantic Type vs. Semantic Role

[[Human]] fire [[Firearm]] {at [[PhysObj = Target]]}

[[Human]] fire [[Projectile]] {at [[PhysObj = Target]]}

Bond walks into our sights and fires his pistol at the audience

The soldier fired a single shot at me

The Italian authorities claim that three US soldiers fired at the car .

– ‘audience’, ‘me’, and ‘car’ have the semantic type [[Human]] and [[Vehicle]] (< [[PhysObj]]).

– The context assigns them the semantic role Target.

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Lexical sets don’t map neatly onto semantic types

• calm as a transitive (causative) verb (pattern 1):• What do you calm? 1 lexical set, 5 semantic types:

– him, her, me, everyone: [[Human]]

– fear, anger, temper, rage: [[Negative Feeling]]

– mind: [[Psychological Entity]]

– nerves, heart: [[Body Part]] but not toes, chest, kidney)

– breathing, breath: [[Living Entity Relational Process]] (but not defecation, urination)

• words from at least 3 of these types are canonical members of the set of things that get calmed

Populating a semantic type with lexical items

Pattern: • [[Human 1 | Event]] calm [[Human 2 | Animal]]

– Canonical lexical items for [[Human]]:

– him, her, me, everyone, ...

– Attributes of [[Human 2]] in this context:

– fear, anger, temper, rage; mind; nerves, heart; breathing, breath

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Why don’t ontologies help WSD?

• Ontologies such as Roget and WordNet attempt to organize the lexicon as a representation of 2,500 years of Aristotelian scientific conceptualization of the universe.

• This is not the same as investigating how people use words to make meanings.

• Why ever did we think it would be?

Why WSD can’t be done(as currently formulated)

• Because words (in isolation) don’t have meanings.

• You’re looking for something that does not exist. – Words have meaning potentials.– The meaning potential of a word is activated by

context (real-world context of utterance and co-text in a discourse).

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What should be done instead• Compare each actual usage with an inventory of

norms. • Best match wins.• Don’t look for the meaning of the word -- look for the

meaning of the pattern.• Distinguish conventional, prototypical usage of words

(norms) from creativity (exploitations).• To do this, we need an inventory of patterned norms.• The Pattern Dictionary of English Verbs will be such

an inventory.