This research brief estimates the overall extent of test measurement error and how this varies across students using New York City student- level longitudinal data across grades 3-8 from 1999- 2007. Results reinforce the importance of accounting for measurement error, as it meaningfully increases effect size estimates associated with teacher attributes. There are important differences in teacher effectiveness that are systematically related to observed teacher attributes. Such effects are important in the formulation and implementation of personnel policies. Also, effect sizes as traditionally measured have led analysts to understate the magnitudes of effects because the standard deviation of observed scores overstates the dispersion of true achievement in the student population.