Uncertainty in Rank Estimation
Implications for Value-Added Modeling Accountability Systems
Published In: Journal of Educational and Behavioral Statistics, v. 27, no. 3, Fall 2002, p. 255-270
Posted on RAND.org on December 31, 2001
Accountability for public education often requires estimating and ranking the quality of individual teachers or schools on the basis of student test scores. Although the properties of estimators of teacher-or-school effects are well established, less is known about the properties of rank estimators. The authors investigate performance of rank (percentile) estimators in a basic, two-stage hierarchical model capturing the essential features of the more complicated models that are commonly used to estimate effects. The authors use simulation to study mean squared error (MSE) performance of percentile estimates and to find the operating characteristics of decision rules based on estimated percentiles. Each depends on the signal-to-noise ratio (the ratio of the teacher or school variance component to the variance of the direct, teacher- or school-specific estimator) and only moderately on the number of teachers or schools. Results show that even when using optimal procedures, MSE is large for the commonly encountered variance ratios, with an unrealistically large ratio required for ideal performance. Percentile-specific MSE results reveal interesting interactions between variance ratios and estimators, especially for extreme percentiles, which are of considerable practical import. These interactions are apparent in the performance of decision rules for the identification of extreme percentiles, underscoring the statistical and practical complexity of the multiple-goal inferences faced in value-added modeling. Our results highlight the need to assess whether even optimal percentile estimators perform sufficiently well to be used in evaluating teachers or schools.