A new study by a public and labor economist suggests that "value added" methods for determining the effectiveness of classroom teachers are built on some shaky assumptions and may be misleading. The study, due to be published in February in the "Quarterly Journal of Economics," is the first of a handful of papers now in the publishing pipeline that are widely seen to be contributing important evidence, both pro and con, regarding the use of value-added assessments, which gauge the effectiveness of schools and teachers by measuring the gains that their students make on standardized tests over the course of a school year. A small number of states have used value-added measures since the early 1990s to track how well their schools are doing. But the method is coming in for new scrutiny amid growing interest from politicians in performance-pay plans for teachers.