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.
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. 11. The article of manufacture of claim 10 , wherein determining the respective compressed word frequencies based on the respective word frequencies comprises: raising the respective word frequencies to a power to form the respective compressed word frequencies, wherein the power is less than 1.