Eric Hanushek, an economist at Stanford, estimates that the students of a very bad teacher will learn, on average, half a year’s worth of material in one school year. The students in the class of a very good teacher will learn a year and a half’s worth of material. That difference amounts to a year’s worth of learning in a single year.It's a good article, focused on how hard is to predict new hires performance in some fields.
Unfortunately, I don't think this kind of metrics are obtained in the education system in my country. In fact, several teacher unions here consider teaching evaluation as something to oppose as it promotes competition over collaboration. Yep. Feedback? Who needs it?