Value added as a measurement tool has been mired in unresolvable controversy, though, because there has been strong disagreement over whether researchers have succeeded in extracting the teacher-specific component to student performance gains, and whether high value-added teachers do anything beyond giving a short-term boost to test scores. Do the gains persist with time? And do teachers that boost test scores actually improve the longer-term life prospects of their students?
The authors of the new study—Raj Chetty and John Friedman of Harvard along with Columbia professor Jonah Rockoff—are the first to link teacher value-added measures to outcomes beyond the classroom, using data on 2.5 million students from a large urban school district who attended grades three through eight during 1989-2009. The two decades of test score history allowed the researchers to generate value-added estimates for district teachers. And since they were able to follow both teachers and students over so many years, the economists were also able to see what happened to test scores when a teacher got transferred to a new school. They found that the arrival of a high value-added teacher resulted in a noticeable bump up in student performance at the new school, and a significant drop in student test scores at the school she had just left. The pattern was reversed for low value-added teachers. These striking patterns (look at Figure 3 in the paper—no advanced math required) put to rest at least some of the concerns about whether value added scores measure teacher quality, or something else.
Even more critical, the researchers were able to link a teacher’s value-added rating to their students’ outcomes later in life. It turns out that the effects of high value-added teachers in grade school continue to reverberate into adulthood. A student who lucks into the classroom of a teacher who is in the top quarter of the district’s instructors is about 0.5 percentage points more likely to attend college than a student of a middle-of-the-road teacher—all the result of a single year of grade school education. If this doesn’t sound like a big number, multiply by 30, which is the average number of students in each classroom, and note that in this school district, only about a third of students attend college in the first place. There are similarly sizeable effects on teenage motherhood, college quality, and retirement savings.
Most strikingly, the study connects teacher value added to earnings in young adulthood. Having a high value-added teacher boosts incomes by about 1 percent relative to a mediocre instructor. Multiplying over a lifetime of earnings, and a classroom full of students benefiting from good instruction, the authors calculate that great teachers create more than a quarter of a million dollars in extra income for students in each of their classrooms.
Before concluding that great teachers should get six figure bonuses, rather than the relatively measly windfalls handed out to a select few D.C. teachers, a number of caveats are in order. First, one of the main critiques of evaluation based on student test scores is that it motivates instructors to “teach to the test.” Student test scores may be a useful measure of teacher quality; but when they’re actually used to evaluate and reward teachers, tests might start having a negative effect: Teachers who are too focused on the objective of maximizing end-of-year test outcomes may erase any long-term benefits. If the incentives were strong enough, teachers might even doctor their students’ scores for a bigger bonus or to save their own jobs. And it’s an open question as to whether such bonuses would keep the very best teachers in the classroom—the burnout rate of inner city teachers is notoriously high, and those who do stick it out are motivated at least in part by a higher calling than a $10,000 bonus check. On the other hand, the extra cash and recognition surely help.
The new study clearly isn’t going to end the debate on the use of value added measures in evaluating teachers, nor should it. The authors are careful to caution their audience about how the link between teacher value added and students’ life outcomes might change once testing becomes high stakes for teachers themselves. Yet keeping such caveats in mind, this study will hopefully provide some clear-eyed analysis that can move forward the often contentious discussion on how to improve American education.