Using generalizability theory to evaluate the reliability of child and adolescent measures enables researchers to enhance precision of measurement and consequently increase confidence in research findings. With an observer-rated measure of child self-regulation, we illustrate how multiple sources of error variance (e.g., raters, items) affect the dependability (replicability) of scores and demonstrate methods for enhancing dependability of observer ratings. Using ratings of 181 children, we illustrate the use of two-facet (i.e., raters and items as sources of error) and three-facet (i.e., raters, items and occasions) analyses to optimize design features of future studies using this measure. In addition, we show how generalizability theory provides a useful conceptual framework for thinking about determinants of scores on acquaintance (e.g., teacher or parent) ratings, as well as observer ratings, and sheds light on the strengths and limitations of both types of data for child and adolescent clinical research. (Contains 7 footnotes, 9 tables, and 2 figures.)