Researchers often develop instruments using correctness scores (and a variety of theories and techniques, such as Item Response Theory) for validation and scoring. Less frequently, observations of children's strategies are incorporated into the design, development, and application of assessments. We conducted individual interviews of 833 prekindergartens to Grade 3 children using 70 length measurement assessment items, recording both correctness and strategy use. We performed qualitative analyses of children's strategies to validate or refine our hypotheses of how items align with strategies, and the concepts and procedures underlying them. We then used these analyses to refine: (a) our definitions of those strategies, concepts, and procedures; (b) the items in the assessment instrument; and (c) the alignment of both. The results will form the foundation of a Computer-Adaptive Test for length measurement, which we intend to validate in future research.