Is Teacher Quality Really Causing the Achievement Gap?

Yesterday, the NY Times released the value-added scores of thousands of teachers over the past five years.  Before and immediately after the release, people seemed to mostly argue the merit of the decision to release the data.  But I have a substantive question about the data.

What caught my eye was that, according to the NYT analysis of this particular set of scores, good teachers are evenly distributed between high-poverty and low-poverty schools.  From the article:

there was no relationship between a school’s demographics and its number of high- or low-performing teachers: 26 percent of math teachers serving the poorest of students had high scores, as did 27 percent of teachers of the wealthiest.

The LA Times reported a roughly a similar situation in LA when they released teachers' scores a couple years ago.  Which is really quite shocking in a number of ways.  Most notably, researchers and practitioners have long assumed that lower-poverty schools had worse teachers than higher-poverty schools -- past studies have repeatedly found that teachers in high-poverty schools are less experienced, turn over at a much higher rate, score lower on achievement tests, attend less selective colleges, etc.  Accordingly, at least part of the theory of action behind the teacher quality movement has been that giving low-income students teachers who are as good as or better than those in higher-income schools would significantly narrow the achievement gap.

But these two measures of teacher quality indicate there may be no major differences between low- and high-poverty schools, while we know that large gaps in achievement still exist between low-income and high-income students.  Which means at least one of two things.

1.) Differences in teacher quality are not a major driver of the achievement gap.

2.) These value-added scores are not a good measure of teacher quality.

I don't think anybody seriously doubts -- or at least that anybody serious doubts -- that some teachers are much better than others and that the best teachers can make a large difference.  But if quality teachers, according to these value-added measure, are roughly evenly distributed between high- and low-poverty schools in LA at the same time that we see differences between high- and low-income students growing, then  improving the quality of teachers (again,as indicated by these value-added measures) in high-poverty schools seems unlikely to close the achievement gap.  Either other factors influence achievement far more, the effects of quality teachers on students are much less direct than many assume, or what we're measuring isn't what matters.

In short, these data indicate that we need to broaden our focus beyond teacher quality and/or re-evaluate the way we're currently measuring teacher quality.