May 10, 2012
National dialogue regarding the reform of public education has often centered on the evaluation of teachers. But performance ratings based on students' test score gains, called value-added (VA) measures, have proved controversial, say Raj Chetty, professor of economics at Harvard University, John N. Friedman, assistant professor of public policy at Harvard Kennedy School, and Jonah E. Rockoff, associate professor of business at Columbia University's Graduate School of Business.
Critics contend that the measures are a poor indicator of teacher quality and should play little, if any, role in high-stakes decisions. However, Chetty, Friedman and Rockoff find that teachers with high VAs contribute enormously to students' educational outcomes, and that this translates into significant lifetime benefits.
- They found that a one-standard-deviation increase in teacher VA corresponds to increases in student math and English scores of 12 and 8 percent of a standard deviation, respectively.
- This corresponds, roughly, to three full months of additional instruction.
- One year of instruction with a teacher in the top 5 percent of VA ratings resulted in immediate end-of-year test scores gains of 4 percent of a standard deviation.
Perhaps more importantly, the researchers found that over their lifetimes, students benefitting from exposure to high-VA teachers saw substantial economic gains.
- A student assigned to a teacher with a VA one standard deviation higher is 0.5 percentage points more likely to attend college at age 20.
- Students of higher VA teachers also attend higher quality colleges, as measured by the average earnings of previous graduates of those colleges.
- A single year in the classroom of a teacher with value added that is one standard deviation higher increases earnings at age 28 by $182, or about 1 percent.
- If that 1 percent advantage were to remain stable throughout an individual's career, it would add up to about $25,000 in total earnings.
Non-economic social gains were also measured among students with high-VA teachers.
- Improvements in teacher value added significantly reduce the likelihood that female students will have a child during their teenage years.
- It increased the socioeconomic status of the neighborhoods in which students live in adulthood.
- It was also associated with higher 401(k) savings rates.
Source: Raj Chetty, John N. Friedman and Jonah E. Rockoff, "Great Teaching," Education Next, Summer 2012.
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