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

Example PCFG

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

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

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

Lexicalized Tree

Example (wrong)