File:The Normal Distribution.svg
Compares the various grading methods in a normal, bell-curve distribution. Includes: Standard deviations, cumulative precentages, percentile equivalents, Z-scores, T-scores, standard nine, percent in stanine.

In education, grading on a bell curve (or simply known as curving) is a method of assigning grades designed to yield a desired distribution of grades among the students in a class. Strictly speaking, grading "on a bell curve" refers to the assigning of grades according to the frequency distribution known as the Normal distribution (also called the Gaussian distribution), whose graphical representation is referred to as the Normal curve or the bell curve. Because bell curve grading assigns grades to students based on their relative performance in comparison to classmates' performance, the term "bell curve grading" came, by extension, to be more loosely applied to any method of assigning grades that makes use of comparison between students' performances, though this type of grading does not necessarily actually make use of any frequency distribution such as the bell-shaped Normal distribution.

Other forms of "curved" grading vary, but one of the most common is to add to all students' absolute scores the difference between the top student's score and the maximum possible score. For example, if the top score on an exam is 55 out of 60, all students' absolute scores (meaning they have not been adjusted relative to other students' scores in any way) will be increased by 5 before being compared to a pre-determined set of grading benchmarks (for example the common A>90%>B>80% etc. system). This method prevents unusually hard assignments (usually exams) from unfairly reducing students' grades but relies on the assumption that the top student's performance is a good measure of an assignment's difficulty.

In the U.S., strict bell-curve grading is unusual at the elementary and secondary school levels (both in age-based grade placement and in standardized testing), but common at the university level.

## Benefits and shortcomings

Viewed practically, curved grading is beneficial (to test-givers, not test-takers) because it automatically factors in the difficulty a group of test-takers had with a test. If the majority of students have high (or low) scores then the middling grade will be adjusted there and higher or lower grades awarded based on this performance. In addition, the curve ameliorates the problem of deciding grades that fall very near a grade margin. Clustering of marks establish where the margin should be placed.

However, grading in this way is essentially normative; scores are referenced to the performance of group member. There must always be at least one student who has a lower score than all others, even if that score is quite high when evaluated against specific performance criteria or standards. Conversely, if all students perform poorly relative to a larger population, even the highest graded students may be failing to meet standards. Thus, curved grading makes it difficult to compare groups of students to one another.

An additional shortcoming is that many students can easily become confused between their relative and absolute grades

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