A study of 2.5 million students by Harvard and Columbia researchers strongly indicates that individual teacher quality based on student test scores significantly impacts students’ life outcomes. While that should be enough evidence for school districts to consider using testing data to help inform firing, tenure and pay policies, the key will be overcoming teacher union opposition.

Analyzing two decades of test results, the researchers found that improving teacher impact on test scores, referred to as “value added,” even in a single grade raises the probability of students attending college and increases their future earnings. The data also suggests that getting rid of the lowest-performing teachers would significantly raise students’ lifetime incomes. Yet, many teacher unions are likely to be unmoved by these findings.

A recent Connecticut Education Association document says that those advocating using test scores as the primary measure of teacher performance “are wrong.” Last year, the National Education Association passed a resolution saying it was open to using test results to evaluate teachers, but, conveniently, no tests were good enough to fit the bill. When asked about the validity of value-added testing, the head of the N.E.A., Dennis Van Roekel, was quoted as saying, “I’m not worried about that at all. . . . The value-added models do not predict [teacher effectiveness] on an individual basis.” The solid findings of the Harvard-Columbia study seem unlikely to sway people who cling to that idea.

John Friedman, a co-author of the study, may say, “The message is to fire people sooner rather than later,” but the opposition of many powerful unions to using testing data to guide teacher evaluation and personnel decisions means that simply having the knowledge on which to base sound policy won’t be enough. Political strength will be needed to change evaluation practices that routinely give passing marks to teachers both good and bad. But change they must. As Anna Bryson, a school board trustee in Southern California, has observed, “We don’t have the right not to look at a possibility that could improve more children’s lives.”