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There is little doubt that teacher quality is a key determinant of student achievement, but finding ways to identify and reward the best teachers has proven illusive. This research brief considers the stability of value-added measures of teacher effectiveness over time and the resulting implications for the design and implementation of performance-based teacher compensation schemes.
There is little doubt that teacher quality is a key determinant of student achievement, but finding ways to identify and reward the best teachers has
proved illusive. Traditionally, teacher compensation has been based on measurable characteristics of teachers, like years of experience or attainment of advanced degrees.
However, recent research has found at best a weak link between such measures and student performance. These findings, combined with the
increased availability of longitudinal data tracking teachers and their students over time, have lead to efforts to measure teacher quality
and make teacher personnel decisions based on student test scores.
"Value-added" analysis seeks to measure teacher quality by estimating the impact of teachers on student achievement, holding constant other factors that affect current student performance, including contemporaneous student ability and effort, family inputs, peer influences and school quality as well as the prior contributions of family, peer, teacher and school inputs.
While much progress has been made, there is still disagreement over which statistical approaches are best and the extent to which they produce accurate or unbiased measures of a teacher's contribution to student learning.1 Clearly, for any performance-based personnel system to provide the correct incentives and enhance teacher quality, it is necessary for valueadded measures to accurately measure true teacher productivity. Avoiding systematic errors in evaluating teacher performance is not
sufficient, however. If value-added measures of teacher quality are unbiased, yet highly variable, their efficacy in high-stakes personnel decisions will be limited. For example, some have proposed using value-added measures to determine which teachers are granted tenure and which are dismissed after an initial probationary period. If value-added measures vary over time, a tenure policy based on a short time frame could lead to the dismissal of many truly effective teachers and the retention of others who ultimately turn out to be relatively ineffective in boosting student achievement. Similarly, if variability in value-added measures over time leads to wide swings in who is rewarded, teachers will view merit-based pay plans as largely random, greatly reducing any incentive effects of pay-for-performance schemes.
In this brief I consider the stability of valueadded measures, the factors that are associated with the degree of stability and the resulting implications for future research and policy. In a companion policy brief, Dan Goldhaber and Mike Hansen explore the long-run stability of value-added measures and the associated implications for tenure policies. In contrast, I focus on the stability of value-added measures over shorter time spans, across school districts, and over test instruments and consider the implications of teacher effect stability for the design and implementation of performance-based teacher compensation schemes.
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