15. A non-transitory computer-readable storage medium comprising instructions for causing one or more processors to: receive a first typed character from a user; determine first probabilities of a first set of possible word completions based on the first typed character; receive a second typed character from the user; determine second probabilities of a second set of possible word completions based on the first typed character and the second typed character; determine a reduction in entropy from the first probabilities of the first set to the second probabilities of the second set, wherein determining the reduction in entropy comprises: determining the reduction in entropy based on third probabilities of a third set of possible word completions, the third set of possible word completions comprising words in the first set of possible word completions other than words in the second set of possible word completions; and in response to the reduction in entropy exceeding a threshold, cause a candidate word to be displayed from the second set of possible word completions.
15. A non-transitory computer-readable storage medium comprising instructions for causing one or more processors to: receive a first typed character from a user; determine first probabilities of a first set of possible word completions based on the first typed character; receive a second typed character from the user; determine second probabilities of a second set of possible word completions based on the first typed character and the second typed character; determine a reduction in entropy from the first probabilities of the first set to the second probabilities of the second set, wherein determining the reduction in entropy comprises: determining the reduction in entropy based on third probabilities of a third set of possible word completions, the third set of possible word completions comprising words in the first set of possible word completions other than words in the second set of possible word completions; and in response to the reduction in entropy exceeding a threshold, cause a candidate word to be displayed from the second set of possible word completions. 16. The non-transitory computer-readable storage medium of claim 15 , wherein determining the first probabilities comprises: determining, using a word n-gram model, fourth probabilities of a plurality of predicted words based on the first typed character; determining, using a character m-gram model, fifth probabilities of a plurality of predicted characters based on the first typed character; and determining the first probabilities based jointly on the fourth probabilities and the fifth probabilities.