A new Harvard study has found that elementary and middle school teachers who help raise their students’ standardized-test scores have a wide-ranging and lasting positive effect on those students — not only within their academic lives but also into their working lives.

Robert H. Meyer, director of the Value-Added Research Center at the University of Wisconsin-Madison, said:

 “That test scores help you get more education, and that more education has an earnings effect — that makes sense to a lot of people.

“This study skips the stages, and shows differences in teachers mean differences in earnings.”

The new study, which tracked 2.5 million students over 20 years, is, as yet, the most comprehensive look at “value-added ratings,” writes Annie Lowrey at the New York Times.

These ratings measure what impact individual teachers have on student test scores. The report is likely to have an effect on the national debate about whether these ratings matter and how to best to measure the quality of teachers.

Many school districts have begun using value-added evaluations when looking at on hiring, firing and compensating their teachers.

Stanford University Professor Linda Darling-Hammond said:

“There is a widespread consensus among practitioners, researchers, and policy makers that current teacher evaluation systems in most school districts do little to help teachers improve or to support personnel decision making. For this reason, new approaches to teacher evaluation are being developed and tested.

“There is also a growing consensus that evidence of teachers’ contributions to student learning should be a component of teacher evaluation systems, along with evidence about the quality of teachers’ practice.”

Supporters of value-added evaluations say that these measures hold teachers directly accountable and can help improve the educational outcomes of millions of children.

Critics, however, say that it is difficult to isolate the effect of a given teacher away from other factors such as the socio-economic status of the student’s parents.

Those pushing back often cite the fact that, typically, many value-added ratings come with a high margin of error. Though, in their study, the researchers found that when looking looking at an individual’s value-added score for three or four classes, some consistently outperformed their peers.

Eric A. Hanushek, a senior fellow at the Hoover Institution at Stanford, said:

“What this paper and other work has shown is that it’s probably more important than people think. That the variations or differences between really good and really bad teachers have lifelong impacts on children.”

The average effect of one teacher on a single student is modest. All else equal, a student with an excellent teacher would be 0.5 percent more likely to attend college. However, if you replaced a poor teacher with an average one would raise a single classroom’s lifetime earnings by about $266,000, the economists estimate.

Professor Friedman, one of the co-authors, said:

“If you leave a low value-added teacher in your school for 10 years, rather than replacing him with an average teacher, you are hypothetically talking about $2.5 million in lost income.”

In wanting to look at the success of students over a longer period of time outside of their current classes, the economists analyzed information on earnings, college matriculation rates, the age they had children, and where they ended up living.

The results were striking, writes Lowrey.

“Looking only at test scores, previous studies had shown, the effect of a good teacher mostly fades after three or four years. But the broader view showed that the students still benefit for years to come.”

The study found that students with the highest quality teachers would become less likely to become pregnant early and were more likely to enroll in college and get better jobs.

In conclusion, the study authors argued that school districts must use value-added measures in evaluations, and fire those who are scoring the poorest.

“The message is to fire people sooner rather than later,” Professor Friedman said.

With the possible margin of error, some suboptimal personnel moves may occur. But researchers say that even if imperfect, well-calculated value-added scores are an important part of evaluating teachers.

Dr. Hanushek said:

“Very few people suggest that you should use value-added scores alone to make personnel decisions.

“What the whole value-added debate has done is push forward the issue of how to evaluate teachers, and how to use that information.”

The study found no evidence for one piece of conventional wisdom: that having a good teacher in an early grade has a bigger effect than having a good teacher in later grades, writes Lowrey.