1. CS626-460: Language Technology For The Web/Natural Language Processing
PCFG and Syntactic Disambiguation

2. Ambiguity Ambiguity Lexical Syntactic Discourse
Preposition attachment ambiguity
Clause attachment ambiguity

3. Prepositional Attachment Ambiguity
saw
the
boy
with
the
telescope V
NP1 P
NP2
VP1
PP1
Where does PP1 attach?
With NP1 or
With VP1

4. Clause Attachment Ambiguity
Ram told the child that he loved that Shyam came to the ground
Parse 1 S VP S1 S2
Here the clause “that he loved” attaches to the clause “that Shyam came to the ground” VP S3 VP PP NP V NP CP NP V CP NP V P NP
NNP VB DT NN C
PRP
VBZ C
NNP VB P DT NN
Ram
told
the
child
that he
loved
that
Shyam
came to
the
ground

5. Clause Attachment Ambiguity
Ram told the child that he loved that Shyam came to the ground
Parse 2
Here the clause “that he loved” attaches to the clause “told the child” S VP S3 VP VP VP VP PP NP V NP CP NP V CP NP V P NP
NNP VB DT NN C
PRP
VBZ C
NNP VB P DT NN
Ram
told
the
child
that he
loved
that
Shyam
came to
the
ground

6. PP Attachment Disambiguation
PP Attachment Disambiguation Methods
Rule-Based Methods
Statistical Methods
Hindle & Ruth (1980),
Ratnaparkhi (1996)

7. Probabilistic Formulation
Given V NP1 P NP2 where does PP (P NP2) attach?
It attaches to V if:
P (Vattach |V NP1 P NP2) > P (NP1attach |V NP1 P NP2)
and attaches to NP1 if:
P (NP1attach|V NP1 P NP2) > P (Vattach|V NP1 P NP2)

8. Assumptions Assumption – 1 (Independence assumption) Assumption – 2
P (Xattach |V NP1 P NP2 <other words>) = P (Xattach |V NP1 P NP2)
Even then the problem can be complex if NP1 ad NP2 are complex
Assumption – 2
P (Xattach |V NP1 P NP2) = P (Xattach |V N1 P N2)
where N1 is head(NP1) ad N2 is head(NP2)
Example
Ram saw the boy with the telescope.
P (sawattach |saw the boy with the telescope)
= P (sawattach |saw boy with telescope)
Assumption – 3 (used sometimes)
P (Xattach |V N1 P N2) = P (Xattach |V N1 P)

9. Resources Needed Observation
It’s a two class classification problem. So a good candidate would be SVM, BP-NN or perceptron
Training Corpora
In the form of tuples <V, N1, P, a> where a = 1 for Vattach and a = 2 for Nattach V N1 P a
see
boy
with 1
drink
tea
find
box 2

10. EXTRA SLIDES

11. Clause Attachment AmbiguityNP VP NP V NP S1 NP V NP CP S2 NP V NP CP NP VP NP V NP CP NP V CP S3 NP V NP CP NP V CP NP VP NP V NP CP NP V CP NP V PP NP V NP CP NP V CP NP V P NP
NNP VB DT NN C
PRP
VBZ C
NNP VB P DT NN
Ram
told
the
child
that he
loved
that
Shyam
came to
the
ground