One-time testing of subject mastery as a condition of teacher certification, however, would be administratively feasible. Faculties of education and other certifying bodies should be required to guarantee that their graduates are fully competent in every subject area that they are qualified to teach.

Teachers who are already certified, however, should be excused from such testing since teachers are professionals, like doctors and lawyers, and as such they should be evaluated on the basis of their results. They would justifiably be insulted and alienated by requirements to take professional development courses.

As noted above, professional development courses are no guarantee of effective teaching. There are many roads to successful teaching, and the only way to be certain that teachers are competent is to measure how much they have added to their students' learning.

For more than ten years, Tennessee has been using the Tennessee Value-Added Assessment System, an analytical method that compares each students' gains from year to year to a normative sample and then calculates the average gain of the students in each class and each school and compares it to the rate of progress in previous years. In this way, each teacher's relative contribution to his or her students' progress can be determined. This system is a cost-effective way to evaluate not only teachers' subject mastery but also their ability to get it across to their students.

Recent research shows that good teaching matters a lot. According to Eric Hanushek, an economist at the University of Rochester, "The difference between a good and bad teacher can be a full level of achievement in a single school year." This means that efficient feedback mechanisms to help teachers and administrators identify best practices and evaluate performance are crucially important.