I.Introduction

It is generally acknowledged that promoting teacher quality is a key element in improving primary and secondary education in the United States. Indeed, one of the primary goals of the No Child Left Behind law is to have a “highly qualified teacher” in every classroom. Despite decades of research, however, there is no consensus on what factors enhance teacher quality.

We focus here on the relationship between teacher productivity and teacher training, including formal pre-service university education, in-service professional development, and informal training acquired through on-the-job experience. Previous research on teacher training has yielded highly inconsistent results and has fueled a wide range of policy prescriptions. Some studies find that formal education is important and these have been interpreted as support for strengthening existing teacher preparation programs in universities1 and increased expenditures on post-college training. Equally common, however, is the finding that formal education is irrelevant, leading others to argue for the elimination of colleges of education.2

One reason for the uncertainty regarding the effects of teacher training is that all past studies have suffered from one of three methodological problems. First, it is difficult to measure productivity, especially in teaching where a student’s own ability, the influences of a student’s peers, and other characteristics of schools also affect measured outcomes. The problem is exacerbated by the fact that assignment of students and teachers to classrooms is usually not