Author and editor Stephen Sawchuk recently took on some of the more timely and frequently debated questions regarding measuring teacher effectiveness in his research brief titled, "What Studies Say About Teacher Effectiveness."
Drawing on a variety of different research studies, the author concludes that:
The variation of student achievement is predominantly a product of individual and family background characteristics. Of the school factors that have been isolated for study, teachers are probably the most important determinants of how students will perform.
Value-added models appear to pick up some differences in teacher quality, but they can be influenced by a number of factors.
The cumulative effect on students' learning from having a succession of strong teachers is not clear.
Teachers improve in effectiveness at least over their first few years on a job, and characteristics such as board certification and content knowledge in math are linked with student achievement. However, these factors do not explain the differences in teacher effectiveness overall.
Merit pay, in the United States, exclusively focused on rewarding teachers whose students produce gains has not been shown to improve student achievement - though some international studies show positive effects.
The connection between student achievement in unionized vs. nonunionized states is unclear.
Read the full report with research citations here