Monday, September 16, 2013
Measuring the Impacts of Teachers I: Evaluating Bias in Teacher Value-Added Estimates
Are teachers' impacts on students' test scores ("value-added") a good measure of their quality? One reason this question has sparked debate is disagreement about whether value-added (VA) measures provide unbiased estimates of teachers' causal impacts on student achievement.
The authors of this study test for bias in VA using previously unobserved parent characteristics and a quasi-experimental design based on changes in teaching staff.
Using school district and tax records for more than one million children, this study finds that VA models which control for a student's prior test scores exhibit little bias in forecasting teachers' impacts on student achievement. Although teachers have substantial impacts, differences in teacher quality account for a small fraction of achievement gaps across demographic groups, as more than 85% of the variation in teacher VA is within rather than between schools.