Background/Context: Developing strong number sense and additive reasoning in the early grades is a critical foundation for success in school and beyond. Longitudinal studies show that early competence with number and operations predicts later math achievement, reading achievement, high school retention rates, and employability and wages in…

Purpose: A growing trend in instructional improvement efforts is the use of formative assessment informed by research-based developmental trajectories of how students gain deeper understanding of subject matter content over time. This article reports the findings of a large-scale experimental study of an innovative mathematics professional…