Thursday, April 26, 2012

Lasting Impacts of Effective Teachers

Teachers who raise test scores have long-term effects on students’ college enrollment and earnings as adults

A study showing the large impacts that highly skilled teachers have on students’ academic achievement and lifetime earnings is available on the Education Next website, Researchers Raj Chetty and John N. Friedman of Harvard University and Jonah E. Rockoff of Columbia University analyzed school-district data from grades 3–8 for 2.5 million children, and linked those data to information on student outcomes as young adults. Their study has received widespread attention since its release as an academic paper in January, 2012. The article, “Great Teaching: Measuring its effects on students’ future earnings,” is accompanied by four commentaries from experts on the study’s policy implications.

The Chetty, Friedman, and Rockoff study finds that, on average, a 1 standard deviation improvement in teacher value added (equivalent to having a teacher in the 84th percentile rather than one at the median) for one year raises a student’s earnings at age 28 by about 1 percent. They estimate that the effect of such a teacher on an entire class of students is more than a $1.4 million increase in cumulative lifetime earnings.

Relative to the median, a teacher at the 84th percentile increases math and English scores by 12 and 8 percent of a standard deviation, respectively -- equivalent to approximately 3 months of additional instruction. Students of highly skilled teachers are more likely to attend college, attend higher-quality colleges, earn more, live in higher socioeconomic status (SES) neighborhoods, and save more for retirement. They are also less likely to have children during their teenage years.

The authors address three criticisms of value-added (VA) measures of teacher effectiveness that Stanford University education professor Linda Darling-Hammond and her colleagues present in a recent article: that VA estimates are inconsistent because they fluctuate over time; that teachers’ value-added performance is skewed by student assignment, which is non-random; and that value-added ratings can’t disentangle the many influences on student progress.

Chetty and his colleagues show, using quasi-experimental tests, that “standard VA measures are not biased by the students assigned to each teacher.” Using over 20 years of student achievement data, the researchers found that changes in the quality of the teaching staff “strongly predict changes in test scores across consecutive cohorts of students in thd same school, grade, and subject.” The most pronounced effects were seen in the departure of ineffective teachers (bottom 5 percent) and arrival of highly effective teachers (top 5 percent). As a result, they conclude that “value-added metrics successfully disentangle teachers’ impacts from the many other influences on student progress.”

In response to the criticism that teacher impacts on student test scores are inconsistent over time, the authors show that “although VA measures fluctuate across years, they are sufficiently stable” that selecting teachers even based on a few years of data would have substantial impacts on student outcomes, such as earnings.

The study’s policy implications are addressed by Douglas Harris of the University of Wisconsin-Madison; Chris Cerf, acting commissioner of education for the State of New Jersey with Peter Shulman of the New Jersey Department of Education; Dale Ballou of Vanderbilt University; and Eric A. Hanushek of Stanford University’s Hoover Institution. In particular, the discussants point to the importance of finding policies that raise the quality of teaching. The political difficulties of implementing actual policies that reflect value-added evidence are noted. Hanushek observes, nonetheless, that the costs of retaining ineffective teachers “are large enough that failing to address them is simply inexcusable.”