1. A computer-implemented method, comprising: determining, by one or more computers, first word frequencies for existing words and a candidate word in a training corpus, each of the existing words and the candidate word being one or more characters, the candidate word defined by a sequence of characters, wherein the sequence of characters define constituent words that are each an existing word in a dictionary; determining, by the one or more computers, second word frequencies for the constituent words and the candidate word in a development corpus; determining, by the one or more computers, a candidate word entropy-related measure based on the second word frequency of the candidate word and the first word frequencies of the constituent words and the candidate word; determining, by the one or more computers, an existing word entropy-related measure based on the second word frequencies of the constituent words and the first word frequencies of the constituent words and the candidate word; and determining, by the one or more computers, that the candidate word is a new word when the candidate word entropy-related measure exceeds the existing word entropy-related measure.