The two figures below (click for larger versions) are taken from the Brookings report by Gordon, Kane and Staiger: Identifying Effective Teachers Using Performance on the Job. The report has received a lot of attention recently thanks to Malcolm Gladwell's New Yorker article. Both are worth a look if you are interested in education. The top figure shows that certification has no impact on teaching effectiveness. The second shows that effectiveness measured in the years 1 and 2 is predictive of effectiveness in the subsequent year. In this case effectiveness is defined by the average change in percentile ranking of students in the teacher's class. Good teachers help their students to improve their mastery, hence percentile ranking, relative to the average student studying the same material.
It's obvious to me that there is gigantic variation in effectiveness among teachers. Gladwell emphasizes how difficult it is to evaluate teaching capability in initial hiring, and how the single most important impact on overall school effectiveness is due to individual teachers (he also makes the analogy to scouting college QBs for pro football -- it's very hard to predict NFL performance based on college performance). The Brookings paper has many policy suggestions, but the basic idea is that if we were disciplined and data-driven we could easily determine which teachers are good and which ones are not.
New Yorker: ...One of the most important tools in contemporary educational research is “value added” analysis. It uses standardized test scores to look at how much the academic performance of students in a given teacher’s classroom changes between the beginning and the end of the school year. Suppose that Mrs. Brown and Mr. Smith both teach a classroom of third graders who score at the fiftieth percentile on math and reading tests on the first day of school, in September. When the students are retested, in June, Mrs. Brown’s class scores at the seventieth percentile, while Mr. Smith’s students have fallen to the fortieth percentile. That change in the students’ rankings, value-added theory says, is a meaningful indicator of how much more effective Mrs. Brown is as a teacher than Mr. Smith.
It’s only a crude measure, of course. A teacher is not solely responsible for how much is learned in a classroom, and not everything of value that a teacher imparts to his or her students can be captured on a standardized test.
Nonetheless, if you follow Brown and Smith for three or four years, their effect on their students’ test scores starts to become predictable: with enough data, it is possible to identify who the very good teachers are and who the very poor teachers are. What’s more—and this is the finding that has galvanized the educational world—the difference between good teachers and poor teachers turns out to be vast.
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. Teacher effects dwarf school effects: your child is actually better off in a “bad” 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’d have to cut the average class almost in half to get the same boost that you’d 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. The school system has a quarterback problem.
In my experience as a university professor I find that most colleagues think of themselves as above-average teachers, even when they are not. Essentially no "value-added" analysis is ever done, so people can have a 30 year teaching career without ever realizing that they aren't effective in the classroom. I've done many dozens of business presentations, to venture capitalists, technology partners, customers, analysts and even potential M&A acquirers, which has helped me improve my own teaching and communication skills. Despite the business setting such meetings are 90 percent teaching -- trying to convey key points to the audience in a limited time. I'm usually there with a team and my team isn't shy about telling me afterwards what worked and what didn't work, so I've had a lot of honest feedback that most professors never get.
The New Yorker cartoon and article capture some essential aspects of teaching and communication that are not widely understood. The teacher has to be simultaneously on top of the material itself and aware of what the class is doing / thinking / confused about. The big neglected factors in teaching are the ability to be a kind of air traffic controller (or symphony conductor) for the class, and the ability to empathize with (read the mind of) an individual student, to see what, exactly, is confusing them.