I write 
Malcolm Gladwell on how to spot great teachers (and why we should want to):
Eric Hanushek, an economist at Stanford, estimates that the students of a very bad teacher will learn, on average, half a year%u2019s worth of material in one school year. The students in the class of a very good teacher will learn a year and a half%u2019s worth of material. That difference amounts to a year%u2019s worth of learning in a single year. Teacher effects dwarf school effects: your child is actually better off in a %u201Cbad%u201D school with an excellent teacher than in an excellent school with a bad teacher. Teacher effects are also much stronger than class-size effects. You%u2019d have to cut the average class almost in half to get the same boost that you%u2019d get if you switched from an average teacher to a teacher in the eighty-fifth percentile. And remember that a good teacher costs as much as an average one, whereas halving class size would require that you build twice as many classrooms and hire twice as many teachers.Hanushek recently did a back-of-the-envelope calculation about what even a rudimentary focus on teacher quality could mean for the United States. If you rank the countries of the world in terms of the academic performance of their schoolchildren, the U.S. is just below average, half a standard deviation below a clump of relatively high-performing countries like Canada and Belgium. According to Hanushek, the U.S. could close that gap simply by replacing the bottom six per cent to ten per cent of public-school teachers with teachers of average quality. After years of worrying about issues like school funding levels, class size, and curriculum design, many reformers have come to the conclusion that nothing matters more than finding people with the potential to be great teachers. But there’s a hitch: no one knows what a person with the potential to be a great teacher looks like.
A lot of it, he explains, is about feedback -- how and how well the teacher responds to students and engages them in the process of learning.
