["Journal of Educational Measurement, Volume 63, Issue 2, Summer 2026. ", "\nAbstract\nThis study applies backward construct mapping, beginning from empirical Rasch modeling and working toward empirical construct definition, to develop and validate an assessment of inflectional morphological processing. This project was a part of the UCSF Dyslexia Center's Dyslexia Phenotype Project, which is a comprehensive multidisciplinary investigation examining cognitive function across multiple domains (language, visuospatial processing, social cognition, executive function, attention, and emotional processing). We administered 64 items in a 2 × 2 × 2 factorial design (regularity, frequency, word class) to 131 participants (84 with developmental dyslexia, 47 typical readers; ages 7‐16 years) using an oral sentence completion task. Multidimensional Rasch analysis validated unidimensionality (latent correlation = .84) and revealed strong measurement properties (person separation reliability = .92). Regularity (β = 3.63) and frequency (β = .99) accounted for 77.5% of item difficulty variance, confirming dual‐route theory. Wright Map analysis revealed five empirically derived waypoints representing natural boundaries in processing demands: from regular high‐frequency forms to irregular low‐frequency forms.\nDespite a meaningful ability difference between groups (1.0 logit; ∼25 percentage point difference at average item difficulty), both groups demonstrated similar item difficulty hierarchies, supporting measurement invariance. Situated within Wilson's (2005, 2023) BEAR Assessment System, this study demonstrates how beginning from an existing dataset and Rasch measurement modeling (Building Block 4) and working toward empirical construct definition (Building Block 1) can transform ordinal rankings into genuine educational measurement, providing an initial, replicable framework for assessment development and actionable insights for differential instruction in students with developmental dyslexia.\n"]