Improving the quality of education delivered through our public schools can not only boost economic growth but also help to narrow income inequality in the U.S. And the best way to improve education is to identify and promote the most talented teachers.
One way of measuring a teacher's effectiveness has been to see how much his or her students' test scores rise. This kind of "value added" measure is straightforward and can easily be used to weed out bad teachers and promote better ones.
Critics complain, however, that this measurement has two potential flaws: Some teachers' scores may rise not because they have performed so well in the classroom but merely because they have better students. And some teachers may push up their students' scores by teaching to the test, rather than giving students the understanding of concepts that pays off in the long run.
Two important pieces of research rebut both of these concerns, suggesting there are significant benefits to be gained from more aggressive use of value-added and other measures to evaluate teachers. The first study, sponsored by the Bill and Melinda Gates Foundation, looked at student selection. In a remarkable feat, the researchers randomly assigned students to about 1,600 different teachers. The random assignment ensured that any observed improvement in the students' test scores was caused by their teachers.