the cat chased the dog
(S (NP (Det the) (N cat)) (VP (V chased) (NP (Det the) (N dog))))
Number of parses: 1

a dog chased a cat
(S (NP (Det a) (N dog)) (VP (V chased) (NP (Det a) (N cat))))
Number of parses: 1

the dog chased a cat on the mat
(S
  (NP (Det the) (N dog))
  (VP
    (VP (V chased) (NP (Det a) (N cat)))
    (PP (P on) (NP (Det the) (N mat)))))
(S
  (NP (Det the) (N dog))
  (VP
    (V chased)
    (NP (NP (Det a) (N cat)) (PP (P on) (NP (Det the) (N mat))))))
Number of parses: 2

Average number of parses: 1.33333333333333