Schwartz, I. S., Garfinkle, A. N., & Bauer, J. (1998). Communicative outcomes for young children with disabilities.

Topics in Early Childhood Special Education, 18, 144–159.
Abstract: The Picture Exchange Communication System (PECS) has become a widely known and used augmentative system for teaching functional communication skills and potentially providing a bridge to speech acquisition. Unfortunately, although there is a great deal of anecdotal clinical evidence about the PECS, there is little empirical information about its efficacy. Lee Mas...