In a meeting some years ago, teachers in a small school in Maryland were puzzling over a noticeable drop in scores that lasted a single year in each grade and moved up one grade each year, much as a recently consumed rat might be seen moving through a python. One teacher offered this explanation: “That’s Leo.” She was referring to a disruptive student who managed to bring down the performance of every class he was in. Whether or not she was correct in the specifics, I don’t know, but her explanation was reasonable in pointing to sampling error as the likely culprit. When I mention this in one
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