Intelligibility and comprehensibility are assessed at the discourse level through analyses of read and conversational speech. Comprehensibility is influenced by segmental and suprasegmental aspects of speech production and by all aspects of language including syntactic adequacy and complexity and semantic variation and appropriateness. In addition, pragmatic aspects of language production influence the comprehensibility of a message, particularly in conversational interactions.
Speech intelligibility can be assessed through an evaluation of read discourse. We tend to use The Rainbow Passage for this purpose. This task eliminates language formulation and competence factors and focuses attention on the ongoing coordination of respiration, phonation, and articulation in continuous speech. However, it also introduces an oral reading factor.
Read speech can be evaluated for intelligibility using a five-point rating scale, with 1 indicating unintelligible speech and 5 indicating fully intelligible speech. It should be noted that research conducted by Metz, Samar and Schiavetti has found that the write-down intelligibility procedure mentioned above yields a more valid and reliable measure of intelligibility.
The suprasegmental aspects of continuous discourse are evaluated using the NTID Speech and Voice Evaluation Form. This can be done on read and/or spontaneous speech samples. Comparing performance on both tasks can be informative.
You can learn more about rating speech and voice parameters by using the self-instruction module. More information on collecting and analyzing spontaneous speech samples is included in Language Assessment.