Towards a Comprehensive Assessment of Speech Intelligibility for Pathological Speech

Speech intelligibility is an essential though complex construct in speech pathology. It is affected by multiple contextual variables and it is often measured in different ways. In this paper, we evaluate various measures of speech intelligibility based on orthographic transcriptions, with respect to their reliability and validity. For this study, different speech tasks were analyzed together with their respective perceptual ratings assigned by five experienced speech-language pathologists: a Visual Analogue Scale (VAS) and two types of orthographic transcriptions, one in terms of existing words and the other in terms of perceived segments, including nonsense words. Six subword measures concerning graphemes and phonemes were derived automatically from these transcriptions. All measures exhibit high degrees of reliability. Correlations between the six subword measures and three independent measures, VAS, word accuracy, and severity level, reveal that the measures extracted automatically from the orthographic transcriptions are valid predictors of speech intelligibility. The results also indicate differences between the speech tasks, suggesting that a comprehensive assessment of speech intelligibility requires materials from different speech tasks in combination with measures at different granularity levels: utterance, word, and subword. We discuss these results in relation to those of previous research and suggest possible avenues for future research