n this article, Frederick Mosteller, Richard J. Light and Jason A. Sachs explore the nature of the empirical evidence that can inform school leaders' key decisions about how to organize students within schools: Should students be placed in heterogeneous classes or tracked classes? What is the impact of class size on student learning? How does it vary? Since tracking (or skill grouping, as the authors prefer to call it) is widely used in U.S. schools, the authors expected to find a wealth of evidence to support the efficacy of the practice. Surprisingly, they found only a handful of well-designed studies exploring the academic benefits of tracking, and of these, the results were equivocal. With regard to class size, the authors describe the Tennessee class size study, using it to illustrate that large, long-term, randomized controlled field trials can be carried out successfully in education. The Tennessee study demonstrates convincingly that student achievement continues when the students move to regular-size classes in the fourth grade and beyond. The authors suggest in conclusion that education would benefit from a commitment to sustained inquiry through well-designed, randomized controlled field trials of education innovations. Such sustained inquiry could provide a source of solid evidence on which educators could base their decisions about how to organize and support student learning in classes and schools.

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