CHUNKING

Question 1

What is the purpose of Chunking

Answer 1

Moving from individual words to meaningful groups “chunks”

Question 2

What is a noun phrase NP

Answer 2

(or nominal phrase)
performs the same grammatical function as a noun
“my favourite book”
“a big blue whale”

Question 3

What is a verb phrase VP

Answer 3

a syntactic unit composed of at
least one verb and its dependents
“read a good novel”
“we will visit the park tomorrow”
“she is writing a letter’

Question 4

What is a Determiner

Answer 4

often a noun phrase can start with one. They can be:
- Simple lexical items: the, this, a, an…
- Simple possessives: John’s
- Complex recursive versions: John’s sister’s husbund’s

Question 5

What is a Nominal

Answer 5

Contains the head and any pre and post modifiers of the head. Follows the determiner

Question 6

VP/NP in context-free grammars

Answer 6

terminals : words
non-terminals : constituents like VP NP or sentence

Question 7

What are pre-modifiers

Answer 7

• Quanitifiers, cardinals, ordinals: eg three cars
• Adjectives: eg large cars
• There is an ordering: three large cars (not large three)
  ‘Cars’ here is the head

Question 8

What is the head in a NP

Answer 8

Main component that carries the primary meaning of the phrase
Usually a noun, pronoun, or word functioning as a noun

Question 9

What are post-modifiers

Answer 9

• Prepositional phrases: eg from Seattle
• Non-finite clauses: eg arriving before noon
• Relative clauses: eg that serve breakfast
• Nominal → Nominal PP
  Nominal → Nominal GerundVP
  Nominal → Nominal RelClause

Question 10

What is Agreement

Answer 10

constraints that hold among various
constituents that take part in a rule or set of rules

eg determiners and the head nouns in NPs have to agree in their number (Not “This flights”)

A way to deal with this is to add further rules in CFGs for NPs to deal with this:
- SingularNP → SingularDet SingularNom
- PluralNP → PluralDet PluralNom

Question 11

What are the constituents of VPs

Answer 11

English VPs consist of a head verb along with 0 or more following constituents which we call arguments

Question 12

What is subcategorisation

Answer 12

We can subcategorise verbs according to the sets of VP rules they can take part in
We have to formally express these constraints

However for CFGs, this does not scale well, explodes the number of rules

Question 13

What are Treebanks

Answer 13

Corpora in which each sentence has been paired with a parse tree

Instead of paying linguists to write a grammar, pay them to annotate real sentences with parse trees

Then use the annotated data to learn the rules

Question 14

What are probabilistic CFGs

Answer 14

Where each production rule has a probability
Using MLE:
We use the treebank to count how many times a rule A -> a is enacted
divide by the number of times a rule with LHS A is enacted
to get P(a|A)

Each rule A → a is assigned a probability p(a|A)
the sum over all expansions of A must equal 1

Question 15

Probabilistic CFGs for Chunking

Answer 15

Chunking can now be done probabilistically using these rewriting rules

Question 16

When calculating a sentence probaility

Answer 16

• Assume productions for each node are chosen independently
• Probability of derivation is the product of the probabilities of its productions

Essentially follow the path and multiply the probabilities of each branch taken