With all the discussion surrounding the need to fire "bad teachers" these days, I wish we could take a step back and examine how many teachers out there are truly worth firing.
A working paper by Thomas Kane and colleagues sheds some light on the issue. In the paper, they investigate the validity of teacher observations conducted by outsiders and find them to be fairly strongly correlated with both teacher value-added scores and levels of teacher experience. This leads to the conclusion that the observations are fairly good measures of teacher effectiveness.
Included in the paper is this chart showing the spread of teachers who averaged ratings on a 1-4 scale.
What sticks out to me is that the distribution is skewed far to the right -- well over half of the teachers earn at least a 3 on the scale, and there are only a few that score really low. This gibes with TNTP's report on San Francisco, in which fewer than 1% of teachers are rated unsatisfactory, and 94% of principals agree that teachers in their school who are underperforming are rated as such.
In other words, maybe the vast majority of teachers are doing a pretty good job. Which, of course, is not to say that teachers who are failing their students should remain as is forever. Clearly, as in any profession, there are teachers who need to receive more training, work harder, and/or move to another job. But if this is the case (and I'm not going to claim it is based on only one study), then we should be a little more careful with our rhetoric. If most teachers are doing a pretty good job, then we should probably spend a little more time praising them, a little less time decrying the fate of the teaching profession, and refer to bad teachers as the exception rather than the rule.