Pooling data of speakers of different languages in the same trial and comparing speech outcome across trials seems possible if the assessment of speech concerns consonants and is confined to speech units that are phonetically similar across languages. Agreed conventions and rules are important. A composite variable for perceptual assessment of velopharyngeal function during speech seems usable; whereas, the method for hypernasality evaluation requires further testing.
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Lohmander et al. (2009) studied this question.
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