Last month, economists at Harvard and Columbia released the largest-ever study of teachers’ “value-added” ratings—a controversial mathematical technique that measures a teacher’s effectiveness by looking at the change in his students’ standardized test scores from one year to the next, while controlling for student demographic traits poverty and race.
Raj Chetty, John Friedman, and Jonah Rockoff analyzed the test scores and family tax returns of 2.5 million Americans over a 20-year period, from 1989 to 2009. The team concluded that students who have teachers with high value-added ratings are more likely to attend college and earn higher incomes, and are less likely to become pregnant teens.
In a rare instance of edu-wonk consensus, both friends and critics of standardized tests are praising the study as reliable and groundbreaking. Indeed, these findings raise several interesting questions about how to evaluate and pay teachers—one of the most controversial topics in American urban politics. In his annual state-of-the-city speech last Wednesday, New York Mayor Mike Bloomberg cited the new research as he promised annual bonuses of up to $20,000 for teachers rated “highly-effective,” based partially on value-added measures and partially on principals’ judgments. In a move that befuddled many casual observers of the education debate, the New York City teachers’ union, the United Federation of Teachers, immediately opposed the proposal.
If we now know teacher effectiveness has a real, measurable impact on both student academic achievement and life outcomes like teen pregnancy, why aren’t teachers’ unions supporting plans to pay teachers with high value-added ratings more money? Pundits like Nick Kristof and the Daily News editorial page have jumped in to claim the new research justifies merit pay plans like Bloomberg’s, and the one instituted by former chancellor Michelle Rhee in Washington, D.C.
The policy implications of the Chetty, Friedman, and Rockoff paper are, however, far from clear. As the researchers note in their conclusion, their study was conducted in a low-stakes setting, one in which student test scores were used neither to evaluate nor pay teachers. In a little-noted footnote (#64) on page 50, the economists write: