Estimating Speech Recognition Accuracy Based on Error Type Classification

Methods for estimating the speech recognition accuracy without using manually transcribed references are beneficial to the research and development of automatic speech recognition technology. This paper proposes recognition accuracy estimation methods based on error type classification (ETC). ETC is an extension of confidence estimation. In ETC, each word in the recognition results (recognized word sequences) for the target speech data is probabilistically classified into three categories: the correct recognition (C), substitution error (S), and insertion error (I). Deletion errors (D) that can occur at interword positions in the recognition results are also probabilistically detected. By summing these CSID probabilities individually, the numbers of CSIDs and, as a result, the two standard recognition accuracy measures, i.e., the percent correct and word accuracy (WAcc), for the speech data can be estimated without using the reference transcriptions. Two recognition accuracy estimation methods based on ETC are proposed. In the first easy-to-use method, ETC is performed by converting the recognition results represented as word confusion networks into word alignment networks (WANs). In the second and more accurate method, the WAN-based ETC results are refined with conditional random fields (CRFs) using various types of additional features extracted for each of the recognized words. Experiments using English and Japanese lecture speech corpora show that the recognition accuracy can be accurately estimated with the CRF-based method. The correlation coefficient and root mean square error between the lecture-level true WAccs calculated using the reference transcriptions and those estimated with the CRF-based method are 0.97 and lower than 2%, respectively. A series of additional experiments and analyses are also conducted to better understand the effectiveness of the CRF-based method.