Measuring intelligibility in spontaneous speech using syllables perceived as understood

Purpose: Intelligibility, the ability to convey a message by speech, is one of the most important variables in speech-language pathology. The assessment of intelligibility is a challenge especially when it comes to spontaneous speech. The aim of the study was to investigate validity and reliability of a method for assessment of intelligibility, syllables perceived as understood (SPU); a method that is more time-efficient than previous methods based on transcription, as it does not require a master transcript for reference. Method: A group of 20 adult listeners transcribed stimuli consisting of spontaneous speech from 16 children (14 with speech sound disorder and two with typical speech and language development, age 4:4 to 8:1, M = 6:0). Intelligibility was calculated based on these orthographic transcripts, as a) proportion of syllables perceived as understood (SPU) and b) proportion of syllables correctly understood (SCU), with reference to a master transcript. Validity was checked through investigation of the correlation and difference between these two measures. Reliability was analysed with inter-listener reliability by intra-class correlation. Results: The correlation between SPU and SCU (the gold standard intelligibility score) was strong and statistically significant, with SPU being consistently higher than SCU. Inter-listener reliability for single measures of intra-class correlation of the assessment by syllables perceived as understood was moderate to low, whereas the inter-listener reliability for average measures of intraclass correlation was high. Conclusions: The method based on SPU might be used for assessment of intelligibility if the median from several listeners is used or when comparing results from the same listener over time. The SPU method might therefore be a valuable tool in a clinical and research context as a more valid option than rating scales and a more time-efficient method than the gold standard SCU method. However, it should be noted that the reliability of the SPU is not as high as for the SCU.