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 un-biased estimates of teachers ’ causal impacts on student achievement. We test for bias in VA using previously unobserved parent characteris-tics 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 ex-hibit little bias in forecasting teachers ’ impacts on student achievement. How can we measure and improve the quality of teaching in primary schools? One prominent but controversial method is to evaluate teachers based on their impacts on students ’ test scores, commonly termed the “value-added ” (VA) approach.1 School districts from Washington D.C. to Los Angeles have begun to calculate VA measures and use them to evaluate teachers. Advocates argue that selecting teachers on the basis of their VA can generate substantial gains in achievement (e.g., Gordon, Kane, and Staige