A new study out by Tom Dee and his colleagues follows on the heels of a prior evaluation of District of Columbia Public Schools' (DCPS) IMPACT teacher evaluation system, which found largely positive outcomes for the system. This time around, they examined the effects of teacher turnover on student achievement. The new focus is presumably prompted by IMPACT, a multifaceted evaluation system that measures student growth, classroom practice (via observations), and teacher professionalism. Teachers receive scores that range from “ineffective” to “highly effective”; the former are “separated” from the district, while the latter are eligible for one-time bonuses of up to $25,000 and a permanent increase in base pay of up to $27,000 per year.
This evaluation, using data from 2009–10 to 2012–13, covers 103 schools between grades four and eight. It examines achievement at the school level, and then the grade level, for particular years. Analysts examine whether teacher effectiveness and achievement are higher or lower as a result of teachers exiting and entering the system.
The evaluation is a well-designed, quasi-experimental study, so it’s not causal in nature. But like any good analysts, the authors subject their data to a number of checks for “robustness” to rule out the possibility, for instance, that systematic sorting of students occurred in response to the turnover. (The best evidence says that it didn’t.)
The bottom line is that teacher turnover in D.C. was found to have an overall positive effect on math achievement, to the tune of 0.08 standard deviations (SD); the effect on reading was positive (0.05 SD), but the latter is barely statistically significant. This overall effect masks important differences, however. When low-performers leave, for example, achievement grows by 21 percent of a SD in math (which equates to something between one-third to two-thirds of a year of learning, depending on grade level) and 14 percent of a SD in reading.
With respect to turnover among low-performing teachers, it’s interesting to note that more than 90 percent occurs in high-poverty schools. But the exit of these instructors consistently produces large improvements in teaching quality and student achievement in math, as well as smaller improvements in reading. “In almost every year, DCPS has been able to replace low-performing teachers with high-performing teachers who have been able to improve student achievement,” the analysts report. When high-performers leave, on the other hand, it does not influence teacher quality or student achievement; it appears that DCPS is able to recruit replacements who are at least as effective. So whereas other studies show generally negative effects of teacher turnover, this one doesn’t.
It turns out, unsurprisingly, that when you enact a policy intended to change the composition of the teaching workforce, and you also have access to a bunch of money to reward the high-performers, the workforce is strengthened and the students benefit. The question is, can any other place replicate these conditions?
SOURCE: Melinda Adnot, Thomas Dee, Veronica Katz, and James Wyckoff, "Teacher Turnover, Teacher Quality, and Student Achievement in DCPS," NBER (January 2016).
