Grading on the curve or by averages assumes that scores for any large group fall in a predictable pattern. A very small number of students will have the highest and lowest grades, while a larger number will have moderately high and moderately low grades. The largest number of students will have grades that fall in the middle. Plotted as a line graph, these scores assume the shape of a bell. This bell-shaped curve is called a normal distribution. In practice, however, grading according to average performance or on the curve doesn't usually use the exact percentages of the normal distribution.
A teacher can adapt the theory of the curve to grade by averages. In one method, the teacher assigns a particular letter grade to the middle or average score. In the traditional A to F system, the average grade is "C." She then decides what percentage of students should get each grade. For example, she decides that the middle 40 percent receive "C" grades, while 20 percent receive each "B" and "D," and 10 percent receive each "A" and "F." In a pure system, the teacher determines the grade distribution beforehand and sticks to it all semester. When a test or paper is graded, she lists the students with each score high to low. She then assigns grades to the set percentage of students at each level.
A teacher often opts out of a completely symmetrical grading curve. For example, he can choose to have a lopsided curve, giving 15 percent an "A" and only 5 percent a "D." Similarly, 25 percent can receive a "B" and only 10 percent a "D." In addition, the teacher does not have to give the same percentage of students each letter grade on every test. He can decide where to place the divisions at scoring time by observing the clumping together of scores and the natural divisions between groups of students. This is more practical since a small group of scores frequently doesn't mirror a bell curve.
Although college professors often use bell curve grading, elementary and secondary school teachers frequently use simpler systems. In one method, teachers shift all scores based on the top score. First the teacher subtracts the highest score from the total number of points possible. She adds this number to every score and assigns letter grades according to a standard. In a traditional percentage system, for example, 90 percent and up equals an "A." If the highest score is 91 out of 100 points, the teacher subtracts 91 from 100. She then adds the result, 9 points, to each score.
When a teacher adapts grades to average performance, most students pass a difficult test. Teachers, students and parents usually perceive this as an advantage. However, if scores are generally quite high, a student who receives a good score by absolute standards may feel cheated. For example, she may get a "D" for a score of 72 percent if the average score is 85 percent. If a teacher chooses to make the average grade a "B," a "B+" or even higher, grade inflation results. Grading a particular group on a curve also fails to hold the group to absolute standards. If a score of 62 percent is the average for one class, that score receives a "C" even if previous classes averaged 75 percent.