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When Testing Fails

By David B. Cohen

A recent spate of articles and education blogs brings encouraging news of research showing that standardized test scores are an unreliable way to measure the effectiveness of a single teacher. It’s wonderful to hear this from people who are not teachers, but of course, it’s not really news: those of us in the classroom, the real teaching experts, have known this all along. Consider this article a friendly “we-told-you-so.”

I’m not suggesting that test scores are without value, or that teachers can’t be assessed and expected to account for their practices. However, it is vital that everyone who cares about teacher quality understand how many variables, apart from the single classroom, affect measurements of student performance.

Test data are appealing because they go into number-crunching spreadsheets so neatly. However, if we’re going to take a scientific approach to instructional evaluation, how can we tolerate definitive conclusions based on the results of an experiment with dozens of...