2. The method of claim 1 , wherein modifying the initial context-free grammar comprises selecting, based at least in part on a statistic derived from application of the backoff grammar rules, a sub-set of a collection of utterances for transcription, the sub-set being fewer than all of the utterances in the collection.
2. The method of claim 1 , wherein modifying the initial context-free grammar comprises selecting, based at least in part on a statistic derived from application of the backoff grammar rules, a sub-set of a collection of utterances for transcription, the sub-set being fewer than all of the utterances in the collection. 3. The method of claim 2 , wherein the statistic is indicative of a frequency with which one of the backoff grammar rules has been identified as a top recognition.