The need for valid and reliable ways to assess developmental outcomes of preschool children becomes continually more acute as public investments in preschool education grow, with corresponding pressure for programs to demonstrate their ability to influence child outcomes. The authors conducted studies of one assessment widely used in Head Start programs and in Michigan's public preschool program: HighScope's Child Observation Record (COR). As the evaluators contracted by the Michigan Department of Education, the authors had access to COR-Adv1.5 data for more than 10,000 children in the Great Start Readiness Program (GSRP), the state public preschool program. The authors' interest was in identifying how program delivery affects children's growth. To assess the validity, fairness, and reliable use of this assessment, the authors conducted three studies. The authors' findings show that the COR-Adv1.5 captures a general child development domain, with three subdomains: cognitive skills, social and emotional development, and motor skills and movement.