CS4705TodayLexical RelationsHomonymyhttp://www.etymonline.com/Related PhenomenaPolysemySynonymyHow could we find Synonyms and Collocations automatically?HyponomyTropes, or Figures of SpeechSumAmbiguity and Word Sense DisambiguationSupervised WSDSlide 15What Features Are Useful?Slide 17Rule Induction Learners and WSDNaïve BayesSlide 20Slide 21Lightly Supervised Methods: BootstrappingDictionary ApproachesSlide 24Slide 25Disambiguation via Selectional RestrictionsSlide 27Selectional Association (Resnik ‘97)Slide 29Slide 30Evaluating WSDSumming UpCS 4705CS4705Relationships among Words, Semantic Roles, and Word-Sense DisambiguationToday•Lexical Relations–Wordnet•Semantic Role–Review: Semantic Roles–Selectional Restrictions–Selectional Association•Word-Sense Disambiguation–Supervised–Unsupervised–EvaluationLexical Relations•Semantic Networks: Used to represent lexical relationships–e.g. WordNet (George Miller et al)–Most widely used hierarchically organized lexical database for English–Synset: set of synonyms, a dictionary-style definition (or gloss), and some examples of uses --> a concept–Databases for nouns, verbs, and modifiers•Applications can traverse network to find synonyms, antonyms, hyper- and hyponyms…–Available for download or online use–http://www.cogsci.princeton.edu/~wnHomonymy•Homonyms: Words with same form – orthography and pronunciation -- but different, unrelated meanings, or senses–A bank1 holds investments in a custodial account in the client’s name.–As agriculture is burgeoning on the east bank2, the river will shrink even morehttp://www.etymonline.com/•bank1 "financial institution," 1474, from either O.It. banca or M.Fr. banque (itself from the O.It. term), both meaning "table" (the notion is of the moneylender's exchange table), from a Gmc. source (cf. O.H.G. bank "bench"); see bank (2). The verb meaning "to put confidence in" (U.S. colloquial) is attested from 1884. Bank holiday is from 1871, though the tradition is as old as the Bank of England. Bankroll (v.) "to finance" is 1920s. To cry all the way to the bank was coined 1956 by flamboyant pianist Liberace, after a Madison Square Garden concert that was packed with patrons but panned by critics. •bank2 "earthen incline, edge of a river," c.1200, probably in O.E., from O.N. banki, from P.Gmc. *bangkon "slope," cognate with P.Gmc. *bankiz "shelf."Related Phenomena•Homophones (same pron/different orth)Read/red•Homographs (same orth/different pron)Bass/bassPolysemy•Words with multiple but related meanings–They rarely serve red meat.–He served as U.S. ambassador.–He might have served his time in prison.–idea bank, sperm bank, blood bank, bank bank–Can the two candidate senses be conjoined??He served his time and as ambassador to Norway.–Same etymology –Often a domain-dependent specializationSynonymy•Substitutability: different words, same meaning–Old/aged, pretty/attractive, food/sustenance, moneyHow big is that plane? How large is that plane?How big are you? How large are you?•What makes words substitutable – and not?–Polysemy (large vs. old sense)–register: He’s really cheap/?parsimonious.–collocational constraints: roast beef, ?baked beefeconomy fare ?economy priceHow could we find Synonyms and Collocations automatically?•Synonyms: Identify words appearing frequently in similar contextsBlast victims were helped by civic-minded passersby.Public-spirited passersby came to the aid of this bombing victim.•Collocations: Identify synonyms or closely related words that do and don’t appear in similar contextsFlu victims, flu sufferers vs. ?Cold victims, cold sufferers…Roast turkey vs. Baked turkeyHyponomy•General: hypernym (super…ordinate)–dog is a hypernym of poodle–Test: ‘That is a poodle’ implies ‘that is a dog’•Specific: hyponym (under..neath)–poodle is a hyponym of dog–Test: ‘That is a poodle’ implies ‘that is a dog’•Ontology: set of domain objects•Taxonomy: Specification of relations between those objects•Object hierarchy: Structured hierarchy that supports feature inheritance (e.g. poodle inherits some properties of dog)Tropes, or Figures of Speech•Metaphor: one entity is given the attributes of another (tenor/vehicle/ground)–Life is a bowl of cherries. Don’t take it serious….–We are the eyelids of defeated caves. ??–GM killed the Fiero. (conventional metaphor: corp. as person)•Metonymy: one entity used to stand for another (replacive)–GM killed the Fiero.–The ham sandwich wants his check. (deferred reference)•Both extend existing sense to new meaning–Metaphor: completely different concept–Metonymy: related conceptsSum•Many definable word relations useful to NLP in different ways–Homonymy, polysemy, synonymy, hypernymy–Homography, homophony–Metaphor, metonymy–Collocations•Resources available to aid in processing –WordNet, FrameNet, online dictionaries,….•A Huge Problem for NLP?Ambiguity and Word Sense Disambiguation•Recall: For semantic attachment approaches: what happens when a given lexeme has multiple ‘meanings’?Flies [V] vs. Flies [N]He robbed the bank. He sat on the bank.•How do we determine the correct sense of the word?•Machine Learning–Supervised methods–Lightly supervised and Unsupervised Methods•Bootstrapping•Dictionary-based techniques•Selectional AssociationSupervised WSD •Approaches: –Tag a corpus with correct senses of particular words (lexical sample) or all words (all-words task)•E.g. SENSEVAL corpora–Lexical sample:•Extract features which might predict word sense–POS? Word identity? Punctuation after? Previous word? Its POS?•Use Machine Learning algorithm to produce a classifier which can predict the senses of one word or many–All-words•Use semantic concordance: each open class word labeled with sense from dictionary or thesaurus–E.g. SemCor (Brown Corpus), tagged with WordNet sensesWhat Features Are Useful?•“Words are known by the company they keep”–How much ‘company’ do we need to look at?–What do we need to know about the ‘friends’?•POS, lemmas/stems/syntactic categories,…•Collocations: words that frequently appear with the target, identified from large corporafederal government, honor code, baked potato–Position is key•Bag-of-words: words that appear somewhere in a context windowI want to play a musical instrument so I chose the