Statistical Natural Language Processing Dr. Besnik Fetahu
Overview • POS tagging • Morphology • Phrase structure and ambiguities • Semantics
POS tagging • Group words of a language into classes which show similar syntactic behavior • These classes are known as grammatical categories or part-of-speech tags • The most important classes are nouns (NN), verbs (VB), and adjectives (JJ) • Nouns: refer to people, animals, concepts, things • Verbs: usually are used to express an action • Adjectives: usually describe properties of nouns
POS tagging Nouns (NN) 𝒕𝒃𝒆 𝒋𝒐𝒖𝒇𝒎𝒎𝒋𝒉𝒇𝒐𝒖 Verbs (VB) The one is in the corner . 𝒉𝒔𝒇𝒇𝒐 𝒈𝒃𝒖 Adjectives (JJ) … adjectives Eat is a verb as it describes Children eat sweet candy . an action, that of children eating candy. Children refers to a group Sweet is an adjective as it of people, whereas candy describes the attribute of refers a type of food . candy.
POS tagging - Ambiguities Nouns (NN) Eat is a verb as it describes Verbs (VB) Children eat sweet candy . an action, that of children Adjectives (JJ) eating candy. Children refers to a group Sweet is an adjective as it of people, whereas candy describes the attribute of refers a type of food . candy. Sweet can be a noun (i.e. in British English) meaning the same as candy Candy can be a verb describing the act of preserving (e.g. fruit).
POS tagging – Open vs. Closed categories • Open or lexical categories: • NN, VB, JJ: constantly there are new additions into the vocabulary of any language, thus, they for each vocabulary they represent the largest groups of words • Closed or functional categories: • PRP, DT: they have few members. The words belonging to these categories have a clear grammatical use.
POS tagging - Morphology • Words categories are systematically related by the process known as morphological processes , which include: • Inflection • Derivation • Compounding • Word morphology deals with word form change • Dependent on the language word forms may vary greatly. E.g. in Finish a verb can have up to 10,000 verb forms
POS tagging - Inflection • Are the systematic modifications of a root by means of prefixes and to indicate grammatical distinctions like singular and plural. • It does not change the class the word belongs to. • It may vary features such as: tense, number, and plurality.
POS tagging - Derivation • Derivation is less systematic. • It usually results in the word changing its syntactic class: • E.g., for wide vs. widely , change from adjective to adverb • In some cases, for some adjectives we cannot derive the adverbial form (e.g. old , difficult ). • Changing adjectives to verbs through the suffix en • Changing verbs to adjectives through the suffix able
POS tagging - Compounding • Compounding refers to the process of merging two or more words together • Most frequent is noun-noun compounds • Other cases include adjectives and verbs: • Over take • Mad cow disease Donau dampf schif fahrts gesellschaft ??? Rind fleisch etikettierungs überwachungs aufgaben übertragungs gesetz
Nouns • Nouns refer to entities in the real world, e.g. people, things, animals etc. • In English, nouns have only one inflection: plural vs. singular • In plural, usually the nouns have the suffix s • English does not have a system for gender inflections • The genitive describes the possessor . E.g., the phrase “the woman’s house” indicates that the woman owns the house. number Singular, plural gender Feminine, masculine, neuter case Nominative, genitive, dative, accusative
Pronouns • Pronouns act like variables in that they refer to a person or thing that is somehow salient in the discourse context. • Pronouns distinguish the number of their antecedent, they also mark person (1st = speaker, 2nd = hearer, or 3rd = other discourse entities). • In the English language, pronouns change when they are in the function of a subject (nominative) or object (accusative) in a sentence.
Words that accompany nouns: Determiners and adjectives • Determiners describe a particular reference to a noun. • ``The’’ article indicates something or someone that is known or that we can uniquely determine. • ``A’’ or ``An’’ indicates a person or thing that was not previously mentioned. • ``This`` or ``That’’ are demonstrative determiners • Adjectives describe properties of nouns • Such uses are usually referred to as attributive or adnominal (e.g. “a red rose” ) • Morphological modifications: • Comparative e.g. richer, smarter • Superlative e.g. richest, smartest • Some cases (e.g. periphrastic ) comparatives and superlatives are formed with auxiliary words (e.g. more or most)
Words that accompany nouns: Determiners and adjectives • Quantifiers are words that express ideas like “all” , “many” , “some” • Interrogative pronouns and determiners • Used in questions and relative clauses • E.g. “which” (interrogative determiner), “whose” (interrogative pronoun)
Verbs • Verbs are used to describe actions, states, activities • The base form of a verb is in present tense (walk) • Infinitive is formed with the base form with to ( to walk) • Progressive uses the suffix –ing and it indicates that an action is in progress • Suffix –ed is used as a suffix for past tense, but also helps in forming the present/past perfect (e.g. has walked, had walked ) • Modals express possibilities or obligations
Adverbs • Adverbs act similarly as adjectives, in that they modify verbs • Adverbs specify place, time, manner or degree • E.g. ”She often travels to Las Vegas” or • E.g. “ She started her career off very impressively” • Some adverbs modify adjectives • E.g. “ a shockingly frank exchange”
Prepositions • Prepositions express spatial relationships • E.g. “ in the glass”, “ on the table”, “ over their heads” • Particles are a subclass of prepositions that can enter into strong-bonds with verbs in the formation of phrasal verbs. • E.g. “ The plane took off at Sam. ” • E.g. “He put me off ” • In some cases, we need to know the meaning of the sentence to distinguish particles and prepositions • E.g. “She ran up a hill” ( preposition ) vs. “She ran up a bill” ( particle )
Conjunctions and Subordinating Conjunctions • Coordinating conjunctions coordinate two words or phrases of (usually) the same category : • husband and wife [nouns] • She bought leased the car. [verbs] the green triangle and the blue square [noun phrases] • She bought her car, but she also considered leasing it. [sentences] • Another function of coordinating conjunctions is to link two sentences (or clauses), e.g. : • She said that he would be late. [proposition] • She complained because he was late. [reason] • I won’t wait if he is late. [condition] • She thanked him although he was late. [concession] • She left before he arrived. [temporal]
Phrase Structure • Languages have constraints on word order • Words are organized into phrases, which gives rise to the idea of such groups behaving as constituents • Constituents can be detected by their being able to occur in various positions, and showing uniform syntactic possibilities for expansion. I put the bagels in the freezer. • The bagels, I put in the freezer. • I put in the fridge the bagels (that John had given me). •
Phrase Structure • A whole sentence is given the category S. • A sentence normally rewrites as a subject noun phrase NP and a verb phrase VP .
Noun Phrases • The noun is the head of the noun phrase • Noun phrases are usually the arguments of verbs • Noun phrases normally consist of an optional determiner, zero or more adjective phrases, a noun head, and then perhaps post-modifiers (e.g. prepositional phrases or clausal modifiers) The homeless old man in the park that I tried to help yesterday
Verb Phrases • The verb is the head of a verb phrase • Verb phrases organize all elements of a sentence that depend syntactically on the verb • Getting to school on time was a struggle. • He was trying to keep his temper. • That woman quickly showed me the to hide.
Prepositional and Adjective Phrases • Prepositional phrases (PP) are headed by a preposition and contain a noun phrase complement • PP can appear within all the other major phrase types (i.e., noun phrases and verb phrases) and usually express spatial and temporal locations and other attributes. • Complex adjective phrases are not so common: • She is very sure of herself; He seemed a man who was quite certain to succeed.
Phrase structure grammars • Syntactic analysis of a sentence tells us the meaning of a sentence from the meaning of the words • In English, the basic word order is subject-verb-object The children ( subject ) should (auxiliary verb) eat spinach ( object ). • This order is modified only to express particular ‘mood’ categories. • E.g. In interrogatives (or questions), the subject and first auxiliary verb are inverted Should (auxiliary verb) the children (subject) eat spinach (object)? • In imperatives there is no subject: Eat spinach!
Phrase structure grammars Rewrite rules Derivations of the rewrite rules
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