: This study applies a validation framework of cognition, implementation, and inference (Pellegrino, DiBello, & Goldman, 2016) to the use of a digital classroom assessment tool, Math-Mapper 6-8, for middle grades mathematics, built using “principled assessment design” (Nichols, Kobrin, Lai, & Koepfler, 2016). It reports on the results of a validation study of learning trajectories for student reasoning on the measurement of circles. A validation method for examining and refining item behavior along a learning trajectory using Rasch analysis and stepwise regression is illustrated as a means to improve learning trajectories at scale and over time.