This post is one in a series highlighting MDRC's methodological work. Contributors discuss the refinement and practical use of research methods being employed across MDRC. Part I of this two-part post discussed MDRC's work with practitioners from the Boston Public Schools (BPS) Department of Early Childhood to construct valid and reliable measures of implementation fidelity to an early childhood curriculum. This Part II examines how those data can reveal associations between levels of fidelity and gains in children's academic skills. [For the first report in this series, see "Working with Practitioners to Develop Measures of Implementation Fidelity. Reflections on Methodology" (ED601201).]