Grading on a curve
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In education, grading on a curve (also referred to as curved grading, bell curving, or using grading curves) is a statistical method of assigning grades designed to yield a pre-determined distribution of grades among the students in a class. The term "curve" refers to the bell curve, the graphical representation of the probability density of the normal distribution (also called the Gaussian distribution), but this method does not necessarily use any specific frequency distribution.
One method of applying a curve uses three steps:
- Numeric scores (or possibly scores on a sufficiently fine-grained ordinal scale) are assigned to the students. The absolute values are less relevant, provided that the order of the scores corresponds to the relative performance of each student within the course.
- These scores are converted to percentiles (or some other system of quantiles).
- The percentile values are transformed to grades according to a division of the percentile scale into intervals, where the interval width of each grade indicates the desired relative frequency for that grade.
For example, if there are five grades in a particular university course, A, B, C, D, and F, where A is reserved for the top 20% of students, B for the next 30%, C for the next 30%-40%, and D or F for the remaining 10%-20%, then scores in the percentile interval from 0% to 20% will receive a grade of D or F, scores from 21% to 50% will receive a grade of C, scores from 51% to 80% receive a grade of B, and scores from 81% to 100% will achieve a grade of A.
Consistent with the example illustrated above, a grading curve allows academic institutions to ensure the distribution of students across certain grade point average (GPA) thresholds. As many professors establish the curve to target a course average of a C, the corresponding grade point average equivalent would be a 2.0 on a standard 4.0 scale employed at most North American universities. Similarly, a grade point average of 3.0 on a 4.0 scale would indicate that the student is within the top 20% of the class. Grading curves serve to attach additional significance to these figures, and the specific distribution employed may vary between academic institutions.
The ultimate objective of grading curves is to minimize or eliminate the influence of variation between different instructors of the same course, ensuring that the students in any given class are assessed relative to their peers. This also circumvents problems associated with utilizing multiple versions of a particular examination, a method often employed where test administration dates vary between class sections. Regardless of any difference in the level of difficulty, real or perceived, the grading curve ensures a balanced distribution of academic results.