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Underlying the California court’s decision on June 10, 2014 to reject teacher tenure as unconstitutional is a controversial body of academic research on teacher effectiveness. The argument that won out was that tenure rules often force school districts to retain their worst teachers. Those ineffective teachers tend to end up at the least desirable schools that are packed with low-income and minority students. As a result, teacher tenure ends up harming low-income students who don’t have the same access as rich students to high-quality teaching.

But for this argument to carry weight we have to be able to distinguish good teachers from bad. How can we prove that California’s low-income schools are filled with teachers who are inferior to the teachers at high-income schools?

Dan Goldhaber, a labor economist at the University of Washington, and Eric Hanushek, a senior fellow at the Hoover Institution at Stanford, were two of the expert witnesses who spoke against teacher tenure in Vegara v. California. Both employ quantitative economic analysis in the field of education. They are both big proponents of using value-added measures to determine who is an effective teacher.

California teacher tenure
The nine plaintiffs, including Beatriz Vergara, who brought suit against the state. This slide, without names, was shown in court.

In value-added analysis, you begin by creating a model that calculates how much kids’ test scores, on average, increase each year. (Test score year 2 minus test score year 1). Then you give a high score to teachers who have students who post test-score gains above the average. And you give a low score to teachers whose students show smaller test-score gains. There are lots of mathematical tweaks, but the general idea is to build a model that answers this question: are the students of this particular teacher learning more or less than you expect them to?

Indeed, researchers using this value-added measure approach have sometimes found low-income schools have a high number of teachers who teach students with below-average test score gains.

Many researchers are questioning whether test-score gains are a good measure of teacher effectiveness. Part of the problem are the standardized tests themselves. In some cases, there are ceiling effects where bright students are already scoring near the top and can’t show huge gains year after year. In other cases, struggling students may be learning two years of math in one year, say catching up from a 2nd grade to a 4th grade math level. But the 5th grade test questions can’t capture the gains of kids who are behind. The test instead concludes that the kids have learned nothing. In both of these cases, with top and bottom students, the teachers would be labeled as ineffective.

Morgan Polikoff of the University of Southern California and Andrew Porter of the University of Pennsylvania looked at these value-added measures in six districts around the nation and found that there was weak to zero relationship between these new numbers and the content or quality of the teacher’s instruction. Their research was published in May 2014, after the Vegara trial ended.

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