Are value-added measures accurate?

It’s just one study, but a new study from Harvard and Columbia economists provides powerful evidence that value-added measures can be an accurate tool in evaluating the effectiveness of teachers. As CPE found in its report, Building a Better Evaluation System, although value-added measures are not perfect, they can be valuable tools in identifying highly effective teachers. This study provides even stronger evidence that this is indeed the case.

The report doesn’t dismiss the limitations of value-added that I discussed in my report: most importantly, that value-added scores tend to fluctuate from year to year, although even one year’s worth of value-added data can be useful. However, the Harvard/Columbia report points out that teacher value-added scores averaged over three or four years are accurate measures of a teacher’s true effectiveness. They were able to determine this in two ways:

1) By finding that when a previously identified highly effective teacher transferred to a new school, that school’s achievement improved within the grade the teacher was assigned. Conversely, when a highly effective teacher left a school, the school’s achievement fell in the grade the teacher previously taught.  Furthermore, the change in scores in both instances matched the change predicted based on teacher’s value-added score.

2) Students of highly effective teachers had better life outcomes than students who had average effective teachers. Students of highly effective teachers were less likely to be teenage parents and more likely to go on to college as well as earn higher wages and save more for retirement.  

These are very important findings, especially considering the authors actually set out to show that value-added measures didn’t work — that they had more to do with student motivation or principal selection or other factors outside the control of teachers, as the authors stated in this New York Times article. Yet, they proved themselves wrong. The data not only showed that teachers matter, but that they matter a great deal to their students’ long-term outcomes. They also showed that value-added measures can be an effective and reliable tool administrators and policymakers can use to ensure all students have access to good teachers. – Jim Hull