This is a guest post from Eric Hanushek, the Paul and Jean Hanna senior fellow at Stanford University’s Hoover Institution, in response to Mike Petrilli’s essay, “The ‘teacher effectiveness gap’ was just a myth: 3 implications.”
Let me try to put some of the issues raised by 
Mike Petrilli’s recent post in perspective. Much of the research has found substantial
variation in teacher quality within all schools. It is difficult to
ascertain how much variation there is between schools, but I don't think
answering that question is key to policy.
- We want to improve the quality of teachers everywhere—which in my opinion calls for weeding out the ineffective teachers everywhere.
- Even if little of the variation in teacher quality is between schools, it does not eliminate concerns about what is happening in disadvantaged schools.
- A recent EdTrust West paper—which is great and which tried to analyze the issues in a serious way—finds some substantial differences in average quality (biased against disadvantaged students) in Los Angeles—so if a serious analysis of New York City finds no bias, we will still be left with policy issues outside of NYC.
- It would not make sense to attempt to redistribute good teachers from middle-class to low-income schools, but we can still pursue policies that try to hold top teachers in poor schools. Thus, direct incentives (tied to effectiveness of teachers) would make sense in poor schools. That would be a direct way to increase average quality. Moreover, apropos the recent LA decision, it might be easier to break the bad contract provisions in schools serving predominantly poor kids.
- Even if average teacher quality is the same across middle-class and poor schools, the poor kids in general will score lower because they come with less average inputs from family and neighborhoods—and we have to deal with that as a nation.
- It does not take indirect evidence from the New York Times, crossed with Marguerite Roza’s data, to infer that teacher characteristics are not good proxies of effectiveness. We have much more extensive and persuasive evidence on this.
