For example, he asked, what effect do 5th grade teachers have on their students' test scores in 3rd and 4th grades?
Because it's impossible for even the best teachers to have an impact on students' previous learning, Mr. Rothstein reasons, there should be no impact.
value-added calculations are based on the assumption that students' classroom assignments are random,
teachers' long-run effects on individual students and finds that they tend to decay or fade out after the first year.
The study also found, as Mr. Rothstein did, that the teacher effects faded in their students' performance from one year to the next, which may be the more important issue, according to Mr. Staiger.
"When calculating the potential value of shifting the teacher-effectiveness distribution, we and others have typically assumed that the effects of a strong teacher persist in the children they teach," write Mr. Staiger and Mr. Kane, who is the faculty director of the Seattle-based Bill & Melinda Gates Foundation's project on policy innovation.