1. A method comprising: determining a first compressed word frequency by raising a first uncompressed word frequency to a power, wherein the power is greater than or equal to 0 and less than 1; determining a second compressed word frequency by raising a second uncompressed word frequency to the power; selecting sample utterance-to-text-string mappings from a corpus of utterance-to-text-string mappings based on the first and second compressed word frequencies; and training an automatic speech recognition (ASR) system with the sample utterance-to-text-string mappings.