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Special Topics in Computer Science Advanced Topics in Information Retrieval Lecture 10: Natural Language Processing and IR. Syntax and structural disambiguation Alexander Gelbukh www.Gelbukh.com
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2 Previous Chapter: Conclusions Tagging, word sense disambiguation, and anaphora resolution are cases of disambiguation of meaning Useful in translation, information retrieval, and text undertanding Dictionary-based methods good but expensive Statistical methods cheap and sometimes imperfect... but not always (if very large corpora are available)
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3 Previous Chapter: Research topics Too many to list New methods Lexical resources (dictionaries) = Computational linguistics
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4 Contents Language levels Syntax Dependency approach Constituency-based approach Head-driven approach Grammars and parsing Ambiguity and disambiguation
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5 Language levels Letters are built up into words Words into sentences Sentences into text Each level has its own representation This allows for modular processing A module describes one level or transforms from one level to another
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6 Source of language complexity: 1-D
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8 Linguistic processor translates between representations
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9 General scheme of text processing Linguistic processor uses linguistic knowledge Applied system uses other types of knowledge (e.g., Artificial Intelligence)
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10 Language levels Morphological: words Syntactic: sentences Semantic: meaning Pragmatic: intention...?
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11 Fine structure of linguistic processor
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12 Example of text Science is important for our country. Science is important for our country. The Government pays it much attention. The Government pays it much attention.
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13 Textual representation Text is a sequence of letter. S c i e n c e i s i m p o r t a n t f o r o u r c o u n t r y. T h e G o v e r n m e n t p a y s i t m u c h a t t e n t i o n.
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14 Morphological analysis Morfological analysis
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15 Morphological representation A sequence of words.
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16 Syntactic parsing
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17 Syntactic representation A sequence of syntactic trees.
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18 Syntactic representation What happened? With whom happened?... their details
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19 Semantic analysis Next lecture...
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20 Syntax The structure describing the relationships between words in a sentence Describes the relationships implied by grammatical characteristics not by meaning Often allows for simple paraphrasing John reads the book The book is read by John
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21 Early approach: Dependency syntax Tree Nodes: words Arcs: modified by Modifies means adds details, clarifies, chooses of many... makes more specific Arcs are typed Types are: subject, object, attribute,... Subject Object Recipient Attribute
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22... Dependency syntax General situation: pay More specifically: the one where: who pays is government what is paid is attention to whom it is paid is it More specifically: attention that is much Subject Object Recipient Attribute
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23 Advantages/disadvantages of Dependency Syntax Advantages Solid linguistic base Rather direct translation into semantics Easily applicable to languages with free word order Korean? Russian, Latin This is why solid linguistic base: good for classical languages! Disadvantages No nice mathematical base No simple algorithms
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24 Most popular approach: Constituency (Phrase Structure grammars) Tree Nodes: nested segments of the phrase Cannot intersect, only nested Usually are labeled with part-of-speech names Arcs: nesting In classical approach, arcs are not labeled [ [ Our Government ] [ pays [ much attention ] [ to it ] ] ]
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25 Constituency [ [ Our Government ] [ pays [ much attention ] [ to it ] ] ] Our Government pays much attention to it
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26 Constituency [ [ Our R Government N ] NP [ pays V [ much A attention N ] NP [ to P it R ] PP ] VP ] S R: pronoun NP: noun phrase N: nounVP: verb phrase V: verb PP: prepositional phrase A: adjective S: sentence
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27 Constituency: graphical representation [ [ Our Government ] NP [ pays [ much attention ] NP [ to it ] PP ] VP ] S S VP NP NP PP NP VP NP NP R N V A N P R Our Government pays much attention to it
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28 Phrase structure grammar Enumerates possible configurations at nodes Usually recursive S NP VP NP A NP NP R NP NP P NP NP N VP VP NP PP VP V S VP NP NP PP NP VP NP NP R N V A N P R Our Government pays much attention to it
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29 Context-independency hypothesis A configuration is possible or not, regardless of where it is used Wherever you find VP NP PP, it can be VP Wherever you find NP VP, it can be S If you can put together S that covers all the sentence, it is a grammatically correct description With this, given a suitable grammar, you can List all sentences of a language List only correct sentences of that language List all and only correct structures Correctness means a native speakers intuition
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30 Generative idea Find a grammar to list all and only correct sentences (with their structures) of a language This is a complete description of that language! How can be useful in analysis? Reverse the grammar
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31 Parsing Given a grammar and a sentence Find all possible structures That describe this sentence with this grammar Many methods. Not discussed today. A lot of research. Very fast algorithms Complexity: cubic in the number of words in the sentence (there are better methods, up to 2.8) Problem: combinatorics of variants
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32 Advantages and disadvantages of consitituency approach Advantages Nice mathematics, very well understood Efficient analysis algorithms, very well-elaborated Good for languages with fixed word order English. Chinese? Disadvantages Difficult translation into semantics Bad when it comes to freer word order Even in English! Worse in other languages
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33 Head-driven approaches Combine some advantages of dependency-based and constituency-based approaches Syntax is still fixed-order. But word dependency information is added Easier translation into semantics More linguistically-based How? In each constituent, the main word (head) is marked It modifies the head of the larger constituent [ [ Our Government ] [ pays [ much attention ] [ to it ] ] ]
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34 Syntactic ambiguity I see a cat with a telescope I see [a cat] [with a telescope] I use a telescope to see a cat I see [ a cat [with a telescope] ] I see a cat that has a telescope Nearly any preposition causes ambiguity Dozens, thousands, millions of variants for a sentence! Because their numbers multiply I see a cat with a telescope in a garden at the shore of a river
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35 Ambiguity resolution Syntactic means are not enough Is telescope more related to see or to cat? Statistical methods: is it used with see or cat? Dictionary-based methods: does it share more meaning with see or cat? Path length in a dictionary of semantic relationships Ideally, context should be analyzed, and reasoning applied: I see a cat with a telescope. It keeps the telescope in its left paw. Now no good methods for this.
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36 Shallow parsing Due to the HUGE problems in resolving ambiguity Do not resolve it! Do what you can de well I see [a cat] [with a telescope] [in a garden] [at the shore] [of a river] Better than nothing Can be done well
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37 Evaluation PARSEVAL international contents A practical parser usually gives only one variant Implies disambiguation! Manually built corpora (treebanks) Compare what the program did with what humans did
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38 One of the uses in IR: Lexical ambiguity resolution Syntactic analysis helps in POS disambiguation: Oil is used well in Mexico. Oil well is used in Mexico. Well = ? But does not help in WSD: I deposited my money in an international bank. I live on a beautiful bank of Han river.
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39 Research topics Faster algorithms E.g. parallel Handling linguistic phenomena not handled by current approaches Ambiguity resolution! Statistical methods A lot can be done
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40 Conclusions Syntax structure is one of intermediate representations of a text for its processing Helps text understanding Thus reasoning, question answering,... Directly helps POS tagging Resolves lexical ambiguity of part of speech But not WSD-type ambiguities A big science in itself, with 50 (2000?) years of history
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41 Thank you! Till June 8? 6 pm Semantics
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