Intelligibility Enhancement Assessment and Intervention: a single-case experimental design with two multilingual university students.

Speech-language pathologists (SLPs) may be approached by multilingual speakers wishing to improve their intelligibility in English. Intelligibility is an essential element of spoken language proficiency and is particularly important for multilingual international students given their need to express complex ideas in an additional language. Intelligibility Enhancement aims to improve the intelligibility and acceptability of consonants, vowels and prosody with multilingual speakers who are learning to speak English. This study aimed to describe the Intelligibility Enhancement Assessment and Intervention Protocols and determine whether the intervention changed multilingual university students' English intelligibility. A multiple-baseline single-case experimental design was applied with direct inter-subject replication across two female participants whose home languages were Vietnamese and Putonghua (Mandarin). English intelligibility was assessed at multiple intervals pre, post and during intervention. The intervention protocol consisted of 11 weekly 1-h sessions with an SLP targeting English consonants, vowels and prosody. Following intervention, both participants displayed increased performance across most measures. For example, the Vietnamese participant's percentage of consonants correct (PCC) increased from 62.5% to 85.0% in probe keywords. Effect sizes, when comparing baseline and withdrawal phases, were 5.5 for PCC, 4.6 for final consonants, 2.3 for consonant clusters and 1.6 for syllables indicating improvements in all variables measured. Her speech rate reduced, word stress increased in accuracy and she perceived less difficulty communicating in English. These promising results suggest further testing of the Intelligibility Enhancement Protocols is warranted to determine effectiveness as an intervention for multilingual speakers.