UPDATED

Update, March 23, 2016: Since this study came out, scholars have released several important new studies on teacher retention, including one showing that the overal rate of retention among beginning teachers is actually higher than previously thought. Keep tabs on new stories related to teacher recruitment, retention, and turnover by clicking this highlighted section.

When teachers leave schools, overall morale appears to suffer enough that student achievement declines--both for those taught by the departed teachers and by students whose teachers stayed put, concludes a study recently presented at a conference held by the Center for Longitudinal Data in Education Research.



The impact of teacher turnover is one of the teacher-quality topics that's been hard for researchers to get their arms around. The phenomenon of high rates of teacher turnover has certainly been proven to occur in high-poverty schools more than low-poverty ones. The eminently logical assumption has been that such turnover harms student achievement.

But a couple years back, two researchers did an analysis that showed, counter-intuitively, it's actually the less- effective teachers, rather than the more- effective ones, who tend to leave schools with a high concentration of low-achieving, minority students. It raised the question of whether a degree of turnover might be beneficial, since it seemed to purge schools of underperforming teachers.

When reporting on that study, I played devil's advocate by pointing out that it didn't address the cultural impact of having a staff that's always in flux. The recently released CALDER paper suggests I may have been right in probing this question.

Written by the University of Michigan's Matthew Ronfeldt, Stanford University's Susanna Loeb, and the University of Virginia's Jim Wyckoff, the new paper basically picks up on the same question. Even if overall teacher effectiveness stays the same in a school with turnover, it's well documented that turnover hurts staff cohesion and the shared sense of community in schools, the scholars reasoned. Could that have an impact on student achievement, too?

To find out, they looked at a set of New York City test-score data from 4th and 5th graders over the course of eight years. The data were linked to teacher characteristics.

(All the usual caveats about limitations of test scores apply, of course.)

Among their findings:

• For each analysis, students taught by teachers in the same grade-level team in the same school did worse in years where turnover rates were higher, compared with years in which there was less teacher turnover.

• An increase in teacher turnover by 1 standard deviation corresponded with a decrease in math achievement of 2 percent of a standard deviation; students in grade levels with 100 percent turnover were especially affected, with lower test scores by anywhere from 6 percent to 10 percent of a standard deviation based on the content area.

• The effects were seen in both large and small schools, new and old ones.

• The negative effect of turnover on student achievement was larger in schools with more low-achieving and black students.

"Turnover must have an impact beyond simply whether incoming teachers are better than those they replaced--even the teachers outside of this redistribution are somehow harmed by it," the authors conclude. "Though there may be cases where turnover is actually helpful to student achievement, on average, it is harmful."

They authors call for more research to identify the mechanics of the decline--whether a loss of collegiality, or perhaps a loss of institutional knowledge among the staff due to turnover, is the cause of the lower achievement.