Searching for Effective Teachers with Imperfect Information

Over the past four decades, empirical researchers -- many of them economists -- have accumulated an impressive amount of evidence on teachers. In this paper, we ask what the existing evidence implies for how school leaders might recruit, evaluate, and retain teachers. We begin by summarizing the evidence on five key points, referring to existing work and to evidence we have accumulated from our research with the nation's two largest school districts: Los Angeles and New York City. First, teachers display considerable heterogeneity in their effects on student achievement gains. Second, estimates of teacher effectiveness based on student achievement data are noisy measures. Third, teachers' effectiveness rises rapidly in the first year or two of their teaching careers but then quickly levels out. Fourth, the primary cost of teacher turnover is not the direct cost of hiring and firing, but rather is the loss to students who will be taught by a novice teacher rather than one with several years of experience. Fifth, it is difficult to identify at the time of hire those teachers who will prove more effective. As a result, better teachers can only be identified after some evidence on their actual job performance has accumulated. We then explore what these facts imply for how principals and school districts should act, using a simple model in which schools must search for teachers using noisy signals of teacher effectiveness. The implications of our analysis are strikingly different from current practice. Rather than screening at the time of hire, the evidence on heterogeneity of teacher performance suggests a better strategy would be identifying large differences between teachers by observing the first few years of teaching performance and retaining only the highest-performing teachers.