15. The article of manufacture of claim 10 , wherein selecting the sample utterance-to-text-string mappings from the corpus of utterance-to-text-string mappings based on the compressed word frequencies comprises: determining a first word selection probability for a first word based on a first compressed word frequency of the first word divided by a first word frequency of the first word, wherein the sample utterance-to-text-string mappings include a particular utterance mapped to a particular text string, and wherein the particular text string contains the first word; and selecting the particular utterance based on the first word selection probability.
10. An article of manufacture including a non-transitory computer-readable storage medium, having stored thereon program instructions that, upon execution by a computing device, cause the computing device to perform operations comprising: obtaining respective word frequencies from a corpus of utterance-to-text-string mappings, wherein the corpus of utterance-to-text-string mappings contains associations between audio utterances and respective text string transcriptions of the audio utterances, and wherein the respective word frequencies are based on occurrences of words in the text string transcriptions; determining respective compressed word frequencies based on the respective word frequencies, wherein a first distribution of the respective word frequencies has a higher variance than a second distribution of the respective compressed word frequencies; selecting sample utterance-to-text-string mappings from the corpus of utterance-to-text-string mappings based on the compressed word frequencies; and training an automatic speech recognition (ASR) system with the sample utterance-to-text-string mappings.