The Los Angeles Times recently attempted to answer that question for parents. Using a statistical technique known as “value added” — which estimates the contribution that a teacher made to a student’s test-score gains from the beginning to the end of the school year—the paper analyzed the influence of third-, fourth-, and fifth-grade teachers on the math and reading scores of students in the Los Angeles Unified School District. The results suggest a wide variation in the quality of L.A.’s teachers. The paper promises a series of stories on this issue over the next several months.

The Times has admirably highlighted the importance of using data to evaluate teacher performance, confirming the findings of a wide and growing body of research. Studies show that the difference between a student’s being assigned to a good or bad teacher can mean as much as a grade level’s worth of learning over the course of a school year. While parents probably don’t need studies to tell them who the best teachers are—such information is an open secret in most public schools—academic research helps underscore the inadequacy of the methods currently used to evaluate teacher performance. Even the nation’s lowest-performing school districts routinely rate more than 95 percent of their teachers as satisfactory or higher.

Teacher evaluations yield absurdly positive results because they’re not tied to objective measures of performance. The current system relies on classroom observation, a thoroughly subjective measure. Tenure protections ensure that poorly rated teachers can’t be removed even when they receive poor performance reports. The result? Principals everywhere hand out positive evaluations to undeserving teachers.

Researchers have worked for years to develop statistical techniques capable of measuring a teacher’s independent influence on student proficiency while accounting for the advantages and disadvantages that students bring with them into the classroom. Value-added is to date the most sophisticated methodology. U.S. education secretary Arne Duncan and his boss, President Obama, support using the value-added metric to assess teachers, and states and school districts across the nation are turning to it to develop new teacher-evaluation tools. Washington, D.C.’s school system has put such a plan into action: last month, the district fired 26 teachers, in part based on poor value-added scores.

more… http://www.city-journal.org/2010/eon0826mw.html