The role of inequality in teacher quality

Taken together, there are few observable teacher characteristics that have consistently shown evidence of large and systematic effects on student achievement on standardized tests. The metric offering the most compelling evidence-a teacher's academic aptitude-is seldom available to researchers, particularly those interested in drawing inferences about changes in teacher quality over time.13 Yet each of the attributes cited has been found in at least one rigorous empirical study to have an important effect-if sometimes small-on student achievement. Several observable qualifications or traits-such as teaching experience, content knowledge, NBPTS certification, and race-more often than not show systematic effects on student outcomes. Other qualifications such as advanced degrees may matter more for the academic achievement of black students than for white. Beyond the independent effects of individual teacher characteristics on student learning, there is only limited evidence on how teacher attributes interact or cumulate in the production of education. It may be that students exposed to teachers lacking along multiple dimensions fall behind more than those whose teachers fall short on only one qualification. Further, repeated exposure to inexperienced or underqualified teachers may have cumulative effects, where differences that appear small in any one year of achievement growth compound into a much larger inequalities in acquired skills over time (Sanders and Rivers 1996). In the following section, we consider how a wide range of teacher attributes varies across schools and classrooms of varying racial compositions, and how the distribution of teachers over students has changed over time.