1. Natural Language Processing
Lecture Notes 8
(Though most of this topic covered in class on the board)

2. Example PCFG

3. Ambiguous Sentence P(TL) = 1.5 x 10-6 P(TR) = 1.7 x 10-6
P(S) = 3.2 x 10-6

4. Lack of Sensitivity to the properties of individual Words
Lexical information can play an important role in selecting between alternative interpretations. Consider sentence:
"Moscow sent soldiers into Afghanistan."
NP → NP PP VP → VP PP
These give rise to 2 parse trees

5. PP Attachment Ambiguity
33% of PPs attach to VPs
67% of PPs attach to NPs S
VP NP NP NP VP
VP PP S VP
VP NP NP PP NP NP
N V N P N
Moscow sent soldiers into Afghanistan
N V N P N
Moscow sent soldiers into Afghanistan

6. Lexicalized Tree

7. Example (wrong)