Efficacy data on alpha supplementation
Provision of alphabetic information without concomitant changes in speech signal increase intelligibility up to 24% (Hustad, 1999).
Changes in speech signal result in intelligibility increases of up to 25% (Crow & Enderby, 1989)
When speakers employ alpha supplementation, intelligibility increases by up to 47% (Beukelman & Yorkston, 1977).