Developmental domains, such as cognitive, language, and motor, are key concepts of interest in longitudinal studies of intellectual and developmental disabilities (IDD). Normative scores (e.g., IQ) are often used to operationalize performance on standardized tests of these concepts, but it is the interval-distributed person-ability scores that are intended for the assessment of within-individual change. Here we illustrate the use and interpretation of several Stanford Binet, 5th Edition score types (IQ, extended IQ, Z-normalized raw score, developmental quotient, raw sum score, age equivalent, and ability score) using data from two longitudinal studies of rare genetic conditions associated with IDD. We found that, although normality assumptions were tenuous for all score types, floor effects led to model unsuitability for longitudinal analysis of most types of norm-referenced scores, and that the validity of interpretation with respect to individual change was best for ability scores. [This article was authored on behalf of the Developmental Synaptopathies Consortium.]