1. A method comprising: determining, by a computing system, a reference transcription of a reference utterance, wherein the reference transcription is derived using a strong acoustic model, a language model and a weight vector, and wherein the reference transcription has a confidence level of at least 70%; based on the reference transcription having the confidence level of at least 70%, determining a secondary transcription of the reference utterance, wherein the secondary transcription is derived using a weak acoustic model, the language model and the weight vector, wherein the secondary transcription has a secondary confidence level, wherein the weak acoustic model has a higher error rate than the strong acoustic model, and wherein the secondary transcription is different from the reference transcription; and based on the secondary transcription being different from the reference transcription, updating the weight vector so that transcribing the reference utterance using the weak acoustic model, the language model and the updated weight vector results in a tertiary transcription with a tertiary confidence level that is greater than the secondary confidence level.