The idea that we should "fire bad teachers" has become the mantra of the day, as though anyone was seriously arguing that bad teachers should be kept. No one is. Instead, the real issue is, and has always been, identification.
Those of us who follow the literature about value-added models (VAM) - the statistical models designed to isolate the unique effect of teachers on their students' test scores - hear a lot about their imprecision. But anyone listening to the public discourse on these methods, or, more frighteningly, making decisions on how to use them, might be completely unaware of the magnitude of that error.