There are two related problems with relying on grades for measuring performance. The first, and most important, is they are one-dimensional. The jaggedness principle, of course, tells us that any one-dimensional ranking cannot give an accurate picture of an individual’s true ability, skill, or talent—or, as psychologist Thomas R. Guskey wrote in Five Obstacles to Grading Reform, “If someone proposed combining measures of height, weight, diet, and exercise into a single number or mark to represent a person’s physical condition, we would consider it laughable. . . . Yet every day, teachers
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