POS Formal Grammar

POS Formal
Grammar

Classes

Open

New ones can be created all the time
Noun, verbs, adjective, adverb

Close

Relative fixed membership
Prepositions (new prepositions are rarely coined)
Particles (up,down)
Determiner (the, a, an)
Pronoun (he, she)
Conjunction
Auxiliary verb (can, should, would)
Numerals

POS tagging

Process of assigning lexical class to each word

Penn Treebank Tagset

English has 45-tag

POS = Disambiguation task
Word = Ambigious

POS
Tagging Method

Rule-Based

Stochastic/
Probabilistic

Assign Potential POS Tags
Assign lists of potential POS tags
to each word based on dictionary

Apply rules to eliminate tag
Apply hand-written constraints until
each word has only one possible POS

Eg. DT cannot immediately precede a Verb
The/DT run/VBP

Bayes Rules

$\widehat {t}^n_1$

$= P(t_1^n | w_1^n)$

$= argmax_{\widehat {t}^n_1} \frac{P(w_1^n | t_1^n) P(t_1^n)}{P(w_1^n)}$

Simplified

$= argmax_{\widehat {t}^n_1} P(w_1^n | t_1^n) P(t_1^n) $
$ = \prod_1^n P(w_i | t_i) P(t_1 | t_{i-1}) $

Constituency

Group of words behave like as a single unit/constituent

Evidence

Appear in similar syntactic environments

Noun Phrases can occur before verbs

Preposed or postposed constructions

on September seventeenth

Context Free Grammar

Formal system for modeling constituent structure

Rules/
Productions

Terminal

Symbol that correspond to words in language

Non-Terminal

Symbol that express abstractions

G = (N, Σ, R, S)

N = Set of non-terminal symbol
Σ = Set of terminal symbols
R = Set of rules of productions
S = Start symbol

Sentence-Level
Construction

Imperative

Begin with verb phrase & no subject

Declarative

Subject noun phrase follow by verb phrase

Wh-Subject Question

Yes-No Question

Begin with auxiliary verb

Wh-Non-Subject Question

Same as Yes-No question