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. We 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, we find 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.
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