MAKING ESSAY TEST SCORES FAIRER WITH STATISTICS

As the graded tests were handed back, a crescendo of groans echoed through the classroom. After the initial shock was registered, the long-suffering teacher smiled benignly and stated, "Your poor performance, relative to previous classes, indicates that this form of the test was more difficult than I had anticipated. I’ll have to curve the scores." The students’ relief was palpable. This sort of scene is common. "Curving the scores" is the transformation of the usual rules of correspondence between percent correct and its associated letter grade. In classroom tests the effect of curving almost always allows a score to qualify for a higher grade than would ordinarily be expected. While almost everyone knows this, the question of why teachers grade on a curve is shrouded in mystery. The answer, in its simplest terms, is that we curve adjust test scores to allow fairer comparisons among individuals who take different forms of the test.