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Natural Language ProcessingNatural Language Processing

Lecture Notes 1

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TodayToday• Administration and Syllabus

– course web page

• Introduction

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Natural Language ProcessingNatural Language Processing• What is it?

– What goes into getting computers to perform useful and interesting tasks involving human languages.

– Secondarily: insights that such computational work give us into human languages and human processing of language.

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Natural Language ProcessingNatural Language Processing• Foundations are in computer

science (AI, theory, algorithms,…); linguistics; mathematics; logic and statistics; and psychology

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Why Should You Care?Why Should You Care?

• Two trends1.1. An enormous amount of knowledge is An enormous amount of knowledge is

now available in machine readable form now available in machine readable form as natural language textas natural language text

2.2. Conversational agents are becoming an Conversational agents are becoming an important form of human-computer important form of human-computer communicationcommunication

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Knowledge of LanguageKnowledge of Language

• Words (words and their composition)• Syntax (structure of sentences)• Semantics (explicit meaning of sentence)• Discourse and pragmatics (implicit and

contextual meaning)

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Small Applications Small Applications

• Line breakers• Hyphenators• Spelling correctors• Optical Character Recognition

software• Grammar and style checkers

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Big ApplicationsBig Applications• Question answering• Conversational agents• Text summarization• Machine translation

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NoteNote

NLP, as in many areas of AI:– We’re often dealing with ill-defined

problems– We don’t often come up with perfect

solutions/algorithms– We can’t let either of those facts get in

our way

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Course MaterialCourse Material• We’ll be intermingling discussions

of:– Linguistic topics

•Syntax and meaning representations– Computational techniques

•Context-free grammars– Applications

•Translation and QA systems

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Chapter 1Chapter 1• Knowledge of language• Ambiguity• Models and algorithms• History

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Knowledge of LanguageKnowledge of Language• Phonetics and phonology: speech

sounds, their production, and the rule systems that govern their use

• Morphology: words and their composition from more basic units– Cat, cats (inflectional morphology)– Child, children– Friend, friendly (derivational

morphology)

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Knowledge of LanguageKnowledge of Language• Syntax: the structuring of words

into legal larger phrases and sentences

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SemanticsSemantics• The meaning of words and phrases

– Lexical semantics: the study of the meanings of words

– Compositional semantics: how to combine word meanings

– Word-sense disambiguation•River bank vs. financial bank

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PragmaticsPragmatics• Indirect speech acts:

– Do you have a stapler?

• Presupposition:– Have you stopped beating your wife?

• Deixis and point of view:– Zoe was angry at Joe. Where was he?

• Implicature:-Yes, there are 3 flights to Boston. In fact, there

are 4.* The general was assassinated. In fact, he isn’t

dead.

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DiscourseDiscourse• Utterance interpretation in the

context of the text or dialog– Sue took the trip to New York. She had

a great time there.•Sue/she; •New York/there; • took/had (time)

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AmbiguityAmbiguity• Almost all of the non-trivial tasks

performed by NLP systems are ambiguity resolution tasks

• There is ambiguity at all levels of language

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AmbiguityAmbiguity• I saw the woman with the telescope• Syntactically ambiguous:

– I saw (NP the woman with the telescope)

– I saw (NP the woman) (PP with the telescope)

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““I made her duck”I made her duck”• I cooked waterfowl for her• I cooked waterfowl belonging to her• I create the duck she owns• I caused her to lower her head quickly…

• Part of speech tagging: is “duck” a noun or verb?

• Parsing syntactic structure: is “her” part of the “duck” NP?

• Word-sense disambiguation (lexical semantics): does “make” mean create, lower head, or cook?

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Dealing with AmbiguityDealing with Ambiguity

• Two approaches:– Tightly coupled interaction among processing

levels; knowledge from other levels can help decide among choices at ambiguous levels.

– Pipeline processing

• Most NLP systems are probabilistic: they make the most likely choices

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Models and AlgorithmsModels and Algorithms• Models (as we are using the term

here): – Formalisms to represent linguistic

knowledge

• Algorithms:– Used to manipulate the

representations and produce the desired behavior •choosing among possibilities and

combining pieces

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ModelsModels• State Machines: finite state

automata, finite state transducers• Formal rule systems: context free

grammars• Logical formalisms: first-order

predicate calculus; higher-order logics

• Models of uncertainty: Bayesian probability theory

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AlgorithmsAlgorithms• Many of the algorithms that we’ll

study will turn out to be transducers; algorithms that take one kind of structure as input and output another.

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AlgorithmsAlgorithms• In particular..

– State-space search•To manage the problem of making

choices during processing when we lack the information needed to make the right choice

– Dynamic programming•To avoid having to redo work during

the course of a state-space search– Machine Learning (classifiers, EM, etc)

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State Space SearchState Space Search• States represent pairings of partially

processed inputs with partially constructed answers– E.g. sentence + partial parse tree

• Goal is to arrive at the right/best structure after having processed all the input.– E.g. the best parse tree spanning the sentence

• As with most interesting AI problems the spaces are too large and the criteria for “bestness” are difficult to encode (so heuristics, probabilities)

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Dynamic ProgrammingDynamic Programming• Don’t do the same work over and

over.• Avoid this by building and making

use of solutions to sub-problems that must be invariant across all parts of the space.