Beck, A., Stoner, J., & Bock, S. (2008) Comparison of PECS and the use of a VOCA: A Replication.

Education and Training in Developmental Disabilities, 43 (2), 198‐216.
Abstract: This study compares use of the Picture Exchange Communication System (PECS) and a Voice Output Communication Aide (VOCA) with four preschool children who were either nonspeaking or limited in their ability to speak and did not use an AAC system to communicate functionally. Lee Mas...