1. A system comprising: one or more computers; and one or more computer-readable storage devices, coupled to the one or more computers, on which are stored a language model, the language model including: a collection of n-grams from a corpus, each n-gram having a corresponding relative frequency in the corpus and an order n corresponding to a number of tokens in the n-gram, each n-gram corresponding to a backoff n-gram having an order of n−1, and a collection of backoff factors α k , one backoff factor for each value of k from 2 to N, inclusive, wherein for each value of k from 2 to N, the backoff factors can be used to compute a backoff score S k for any n-gram of length k that is not present in the language model, wherein each n-gram of length k that is not present in the language model has a particular longest backoff n-gram that is present in the collection of n-grams and that has a particular relative frequency f and a particular length m, wherein the backoff score S k is determined as a function of the collection of backoff factors, the particular length m and the particular relative frequency f of the particular longest backoff n-gram.