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

Ambiguity Ambiguity Lexical Syntactic Discourse Preposition attachment ambiguity Clause attachment ambiguity

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

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

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

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

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)

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)

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

EXTRA SLIDES

Clause Attachment Ambiguity NP 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