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New York Times Room for Debate


Imperfect, but Useful

September 07, 2010

By Marcus A. Winters

What are the benefits and pitfalls of using student test scores to measure a teacher’s effectiveness?

As a statistical tool, “value-added” assessment is not a perfect measure of teacher quality. Indeed, there will never be a perfect measure of teacher quality. The real question, then, is whether this analysis can improve the methods we use to evaluate teacher performance. There is ample reason to believe that it can.

An objective measure of what an individual teacher adds to a student’s proficiency.

Currently, nearly all teachers are rated satisfactory or higher. No one honestly believes that this system accurately assesses teacher quality. Uniform satisfactory ratings are inconsistent with empirical research showing wide variation in teacher quality, and they defy common sense.

When evaluating teacher quality, what we really want to know is: To what extent does an individual teacher contribute to her student’s proficiency? Value-added test-score analysis provides us with an objective estimate of this contribution.

It is true that the test-score analysis doesn’t tell us everything we want to know about a teacher. For example, if a teacher is performing poorly, it can’t tell us why. Also, random error ensures some teachers will receive ratings that over- or underestimate their actual influence on student proficiency. For those and other compelling reasons, it would be irresponsible to use only value-added analysis to evaluate teachers.

Nonetheless, imperfect value-added assessment is surely an improvement upon the current system, which makes no meaningful attempt to differentiate teachers by their effectiveness. Further, recent empirical research finds evidence that value-added measures of a teacher’s past performance are far better predictors of her future students’ achievement than are other observed characteristics, like her credentials and years of experience.

Everyone agrees that teacher quality is crucial for student learning. It’s well past time that public schools took seriously the job of identifying it. Value-added assessment is an important and powerful tool that public schools should make use of to ensure that the most effective teachers are educating our kids .

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