Recently, a number of districts have introduced human capital reforms intended to improve the quality of teaching either through compositional change in the workforce or by improving the extant teaching workforce.  There have been several high profile pay for performance pilots employing rigorous experimental methods targeting improvements among existing teachers that have produced largely disappointing results (Fryer et al., 2012; Fryer, 2013; Glazerman and Seifullah, 2012; Springer et al., 2010).  The District of Columbia Public Schools (DCPS) has operated, IMPACT, a uniquely high stakes, at scale, robust teacher assessment and incentive system for five years. During this period, IMPACT has led to the performance-based forced separation of more than 4 percent of the teacher workforce, and hundreds of teachers who voluntarily separated after receiving a low evaluation rating (Dee and Wyckoff, 2013).  Theoretically, increasing the exit rates of poorly performing teachers will result in improved student achievement, provided the average performance of the incoming teachers exceeds that of those exiting.  This paper examines how the churn of teachers associated with IMPACT influences student-level achievement.

Our empirical approach examines how different forms of teacher turnover (e.g., voluntary attrition or forced separation) influence subsequent student outcomes within the affected school-grade cells. This approach parallels methods employed by Chetty, Friedman and Rockoff (2011) as well as Rivkin, Hanushek, and Kain (2005), Jackson and Bruegmann (2009), and Ronfeldt, Loeb, and Wyckoff (2013). At its core, this methodology exploits the teacher turnover that occurs across adjacent cohorts within each school as an exogenous source of variation in teacher quality.  In general, we suspect teacher turnover is likely to be non-random: the unobserved school traits that influence teacher turnover are also likely to influence student achievement. However, the variation isolated by our approach – the year-to-year variation in teacher turnover within given school-by-grade cells – may be a plausibly exogenous determinant of student achievement. Thus, we observe the effects of different forms of teacher turnover on student achievement.  Because teacher turnover during the IMPACT era is closely related to its design features, these results will provide important evidence on whether IMPACT has shaped teacher quality as evidenced by changes in student performance.