19. A computer-readable storage device having instructions stored which, when executed by a computing device, cause the computing device to perform operations comprising: selecting a plurality of language models; for each time period of a plurality of time periods: identifying a first utterance and a second utterance received during each time period, wherein the first utterance was recognized using a first language model of the plurality of language models and the second utterance was recognized using a second language model of the plurality of language models; identifying distinctions between the first utterance and the second utterance for each of the plurality of time periods; determining when a significant word usage change has occurred within the first language model and the second language model by comparing the distinctions to previously recorded distinctions; and when the significant word usage change is detected: identifying a word corresponding to the significant word usage change; generating, from the utterances, a first cluster of utterances comprising the word; generating, from the utterances, a second cluster of utterances not comprising the word; and updating the plurality of language models using the first cluster of utterances and the second cluster of utterances.
19. A computer-readable storage device having instructions stored which, when executed by a computing device, cause the computing device to perform operations comprising: selecting a plurality of language models; for each time period of a plurality of time periods: identifying a first utterance and a second utterance received during each time period, wherein the first utterance was recognized using a first language model of the plurality of language models and the second utterance was recognized using a second language model of the plurality of language models; identifying distinctions between the first utterance and the second utterance for each of the plurality of time periods; determining when a significant word usage change has occurred within the first language model and the second language model by comparing the distinctions to previously recorded distinctions; and when the significant word usage change is detected: identifying a word corresponding to the significant word usage change; generating, from the utterances, a first cluster of utterances comprising the word; generating, from the utterances, a second cluster of utterances not comprising the word; and updating the plurality of language models using the first cluster of utterances and the second cluster of utterances. 20. The computer-readable storage device of claim 19 , having additional instructions stored which, when executed by the computing device, result in preliminary operations to be executed by the computing device before the operations of claim 19 , the preliminary operations comprising: for each time period of a plurality of time periods, computing a matrix of associational usage scores among all words of a list of frequently occurring words; computing differences in the associational usage scores of two of the matrices to produce a difference matrix; producing a set of clusters of utterances based on similarity in associational usage scores of the difference matrix, to yield a produced set of clusters; and creating a plurality of word classes based on a result of producing a set of clusters of utterances, wherein: determining determines a significant word class change, and generating the first cluster comprises generating, from the examined utterances, a cluster of utterances having a word class corresponding to the significant word class change.