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The Handbook of Language and Speech Disorders


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structures and functions because intelligibility is a product of the subsystems working together as a collective whole to produce speech. Further, intelligibility requires the contribution of a listener. It is also influenced by contextual factors. A person’s ability to participate in his or her life situations (the participation level of the ICF‐2) is impacted by intelligibility as well as the broader notion of comprehensibility.

      Intelligibility is challenging to measure, in part because of its complexity. A considerable body of research has demonstrated that there are many variables that may influence intelligibility, as noted above. Operationally, there are two main approaches to measuring intelligibility: objective measures and subjective measures.

       4.3.1 Objective Measures of Intelligibility

      There is a body of evidence showing that adult speakers without communication disorders entrain their speech production behaviors to one another during conversation, essentially becoming more acoustically and perceptually similar (Borrie, Barrett, Willi, & Berisha, 2019; Giles & Powesland, 1975; Pardo, 2006). These interdependent adjustments to speech production behavior occur at a seemingly unconscious level during spoken dialog and are considered to reduce the computational load of spoken language processing and improve the effectiveness and efficiency with which information is exchanged. While this is a new area of investigation for individuals with intelligibility impairment, work has begun to examine entrainment of speech behaviors in the conversations that occur between individuals with dysarthria and adults without communication disorders. Preliminary evidence suggests that, while substantially reduced relative to the entrainment that occurs between two adults without communication disorders, entrainment of some speech behaviors may transpire in conversations with this clinical population (Borrie, Barrett, Liss, & Berisha, 2020; Borrie, Lubold, & Pon‐Barry, 2015). While the link between entrainment and traditional measures of intelligibility has not yet received attention, entrainment of speech behavior, even in conversations with individuals with dysarthria, has been linked with objective measures of improved communicative efficiency.

      One alternative that has been used in the literature is a hybrid approach, combining elements of language sample analysis, the procedures described by Flipsen (2006), and transcription intelligibility of elicited utterances. Hodge and Gotzke (2014) measured intelligibility of spontaneously generated speech by having experts create a master transcript against which unfamiliar listener responses could be scored. They then employed unfamiliar listeners who completed orthographic transcription tasks like those described for elicited words and sentences, above, to yield a percentage intelligibility score (Hodge & Gotzke, 2014). Findings indicate that intelligibility of elicited sentences from the TOCS did not differ from intelligibility of spontaneous speech samples. This convergence of findings may be related to similarities in methods, including use of unfamiliar listeners and use of a listening task that was constrained and decontextualized. Although this type of hybrid approach may not be fully reflective of the rich context available in dynamic interaction between speaker and listener, results provide important construct validity for the use of measures such as the TOCS for understanding intelligibility in children.

       4.3.2 Subjective Measures of Intelligibility

      The second main approach to measurement of speech intelligibility involves subjective measures. Subjective measures of intelligibility generally require listeners to quantify their perception of a speaker’s intelligibility by assigning a number to, or scaling, what they heard (Weismer & Laures, 2002). Direct magnitude estimation (DME) procedures have been used frequently for the study of contributors to intelligibility in dysarthria. DME procedures require listeners to scale the intelligibility of speech relative to a modulus or exemplar. In contrast, use of Likert ratings, or equal appearing interval scales, requires listeners to assign numbers based on perceived similarity