This application is in response to FOA PAR-11-045, Outcome Measures for Use in Treatment trials for Individuals with Intellectual and Developmental Disabilities. We propose to further develop and validate an endpoint measure of communication for use in clinical trials research. We created a scale that reflects the developmental hierarchy of early communication, referred to as the Communication Complexity Scale (CCS). The CCS can be used with children and adults with severe disabilities including individuals with autism, Rhett syndrome, Fragile X syndrome (FXS), and Angelman syndrome. The focus is on developmentally early communication, beginning with alertness to changes in the environment and progressing through beginning word use. The CCS fills a significant gap in available measures that provide a reliable index of early communication development. The CCS has been used successfully in our research projects with over 100 individuals. In Aim 1 we will evaluate the psychometric properties of the CCS with 300 new participants across ages and types of disabilities. We will measure inter-rater and test/retest reliability of the scale, conten validity, concurrent validity, and discriminant validity. In Aim 2, we will provide evidence about the sensitivity of the CCS to measure change in clinical trials for behavioral interventions of varying lengths, with both preschool and early elementary-aged children. Aim 2 will be accomplished in collaboration with researchers at UCLA. In Aim 3 we will investigate the utility of the CCS as an outcome measure. Following the development of an administration and scoring manual in both English and Spanish, usability testing of the scale and training materials will be completed. Through this research, we will be addressing a critical need identified by NIH to develop and validate outcome measures for use in clinical trials by individuals with ID.