Everyone's favorite sound bite

Eduwonkette's post this morning on the long-term effects of teachers needs explanation. In attempting to report on a new study, the crux of her argument is thus:

It's everyone's favorite sound bite: good teachers alone can close racial and socioeconomic achievement gaps. But if the entire teacher effect doesn't persist from year-to-year - that is, a student only retains some fraction of the learning advantage they get from having a highly effective teacher - these claims simply don't hold up.

There are two problems with this argument. One, the entire teacher effect need not persist for it to be real, and two, the evidence shows time and again that placing a student in the hands of an excellent teacher for even one year has lasting impacts.

Let's look at the study itself. Eduwonkette repeats its finding that, "only about one-fifth of the test score gain from a high value-added teacher remains after a single year." If we dig a little deeper, we see that the researchers found a one-year long-term learning coefficient of .66, with the teacher the student had a year beforehand contributing a full one-third of this effect.

If we look two years out--that is, the student had been under the tutelage of two different educators for two additional years of schooling--that original teacher still contributed about one-fourth of the student's long-term learning gains. These gains attributable to teachers come after controlling for incoming student achievement scores, gender, race, age, income, and learner characteristics (disability and limited English proficiency). These findings in no way challenge previous studies indicating teacher effects accumulate over time.

Update: Eduwonkette responds, but she clearly did not read the study carefully. Unlike other value-added studies that look at teacher effects over time with multiple teachers, this one looked at only the learning gains attributable to one teacher in one year. Then, it asked the question of how much, for example, we can attribute a 5th or 6th grader's math and reading scores with his or her 4th grade teacher. In finding that the 4th grade teacher alone accounted for one third and one quarter, respectively, of a student's achievement scores one and two years later, the study gave us more evidence of lasting teacher effects. True, they diminish slightly over time, but without controlling for future teachers, these findings in no way dampen the lessons from previous studies.