School principals are viewed as critical actors to improve student outcomes, but there remain important methodological questions about how to measure principals’ effects. We propose a framework for measuring principals’ contributions to student outcomes and apply it empirically using data from Tennessee, New York City, and Oregon. As commonly implemented, value-added models misattribute to principals changes in student performance caused by unobserved time-varying factors over which principals exert minimal control, leading to biased estimates of individual principals’ effectiveness and an overstatement of the magnitude of principal effects. Based on our framework, which better accounts for bias from time-varying factors, we find that little of the variation in student test scores or attendance is explained by persistent effectiveness differences between principals. Across contexts, the estimated standard deviation of principal value-added is roughly 0.03 student-level standard deviations in math achievement and 0.01 standard deviations in reading.
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