I always get a kick out of seeing headlines like "Big Study Links Good Teachers to Lasting Gain." Maybe I should feel sad: Do people truly find this newsworthy? According to the study, elementary- and middle-school students who have exposure to "good" teachers not only earn more money over the course of their working lives, they are also more likely to enroll in college, less likely to become teenage parents, and grow an average of 3.6 inches taller.
OK, I made that last one up, but, still, the results are impressive.
The controversy arises over what exactly makes a teacher "good." This study looks at "value-added." To put it probably too simplistically, this measures the impact a teacher has on students by looking at student improvement. Thus, if a student scores, say, 60% on a test at the beginning of a semester, and 70% at the end, then the teacher could be said to have "added" ten points to that student's score. Of course, individual student results can vary widely, and there are numerous independent variables other than the teacher that can have an impact on student performance, but if researchers assess an entire class and find an average 10-point increase in student performance on standardized tests, one can make a reasonable argument that the teacher played some part in the students' achievement. The argument becomes stronger if this same teacher consistently achieves this kind of result.
As a teacher, I like the value-added assessment because of its focus on improvement. In a way, it penalizes teachers less for the "quality" of their students than other measurements. If one teacher has a class that scores, on average, 95% on a test, and another has a class that scores 65%, you might conclude that the first teacher is better. On the other hand, if that first teacher's class scored, on average, 94% at the beginning of the semester, while the second teacher's class scored 55%, which teacher did a better job? Of course, the teacher with the "A" students would complain that a value-added system unfairly penalizes her for teaching good students, which shows the importance of multiple-measurements of teacher quality.
At any rate, with the constant attacks on teachers and their unions, it's nice to see some scientific validation of what should be obvious. Good teachers make a difference.