14. The method of claim 1 , further comprising performing, before performing the acts of claim 1 : for each of the 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 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 usage 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.
15. The method of claim 14 , further comprising: prioritizing the first cluster of utterances and the second cluster of utterances.