A recent study conducted by economists from Harvard and Columbia, and reported in The New York Times, however, suggests that good teaching affects long-term student gains even more than most people probably suspected.
Raj Chetty and John N. Friedman (Harvard) and Jonah E. Rockoff (Columbia) followed 2.5 million students over a 20-year period. They found that even a single year under the tutelage of a teacher ranked “excellent” led to gains, albeit modest ones. For example, over the course of his/her life, a student taught by an excellent teacher for one year is likely to earn $4,600 more than a student who had an average teacher during the same year, and is also 0.5 percent more likely to go to college. However, when viewed in the aggregate -- lots of students over many years -- students with excellent teachers were significantly more likely to attend college, earn higher income as adults, and avoid teen pregnancy. These results held true even when the researchers controlled for the kind of socio-economic factors so often identified as the primary causes for student performance.
For those who attempt to examine teacher quality objectively, one of the trickiest, and also most critical, decisions to make is how to measure teaching prowess qualitatively. Chetty, Friedman, and Rockoff used value-added ratings, which measure the impact individual teachers have on student test scores, to categorize teachers as “excellent,” “average,” or “poor.” Value-added ratings are extremely controversial, with many opponents arguing that good teaching cannot truly be measured or that it is not measured by student test scores. Others worry that making value-added ratings a significant part of teacher evaluations will lead to “teaching to the test,” cheating among teachers, or competition between teachers to stack their classes with students who are perceived as smart.
Despite the controversy, the study certainly seems to suggest that teachers’ impact on their students should not be underestimated.