This is more than a week old but I just finished reading it, so stick it. Malcolm Gladwell says that the problem of finding good teachers is the same sort of problem encountered by scouts attempting to find good NFL quarterbacks.
The problem with picking quarterbacks is that [college QB] Chase Daniel's performance can't be predicted. The job he's being groomed for is so particular and specialized that there is no way to know who will succeed at it and who won't. In fact, Berri and Simmons found no connection between where a quarterback was taken in the draft -- that is, how highly he was rated on the basis of his college performance -- and how well he played in the pros.
A group of researchers -- Thomas J. Kane, an economist at Harvard's school of education; Douglas Staiger, an economist at Dartmouth; and Robert Gordon, a policy analyst at the Center for American Progress--have investigated whether it helps to have a teacher who has earned a teaching certification or a master's degree. Both are expensive, time-consuming credentials that almost every district expects teachers to acquire; neither makes a difference in the classroom. Test scores, graduate degrees, and certifications -- as much as they appear related to teaching prowess -- turn out to be about as useful in predicting success as having a quarterback throw footballs into a bunch of garbage cans.
The upshot is that NFL quarterbacking and teaching are both jobs that need to be performed in order to find out if a certain person is good at them or not. For more, check out a follow-up post on Gladwell's blog.