The goal of this study was to evaluate speech understanding outcomes in prelingually deaf children who use a cochlear implant device. Specifically, we discuss investigations on 2 prognostic variables--age at implantation and degree of residual hearing--and use a novel method: binary partitioning analysis. Our outcome measures are standard speech perception evaluations, including the Word Intelligibility by Picture Identification (WIPI) test, the Phonetically Balanced-Kindergarten (PBK) test, and the Glendonald Auditory Screening Procedure (GASP). Regarding age at implantation, we definitely showed that growth rates of speech understanding do relate to age at implantation, but not in a simple fashion. We used binary partitioning in an attempt to find the age at implantation that best separates the performances of children with younger versus older ages at implantation. We found that there is no one "critical age"; much appears to depend on the nature and difficulty (eg, whether open- or closed-set) of the test used. Regarding residual hearing, binary partitioning analysis was unable to show that the amount of residual hearing (as shown by preimplantation audiometric data) has any significant bearing on speech outcome measures in congenitally or prelingually deaf children.