Bell curve grading, also known as grading on a curve, is a system that uses a bell curve to evaluate students' performance. Within this system, each individual's grade is relative to the performance of the rest of the class.
A common method: Find the difference between the highest grade in the class and the highest possible score and add that many points. If the highest percentage grade in the class was 88%, the difference is 12%. You can add 12 percentage points to each student's test score.
Grading on the bell curve system can and does impact grades. It can lower or improve student grades, standardize grades across instructors, and prevent grade inflation.
Bell Curve in performance management, in case you are unfamiliar, is a system of performance appraisal and a method to evaluate performance of employees by force ranking them into a bell shaped graph varying from worst performers to best performers.
If you use a five point scale (similar to grades), many companies say that “no more than 10% of the population gets a rating of 1″ and “10% of the population must be rated a 5.”
Justifications for the Bell Curve often highlight its main purpose of preventing grade inflation from happening within an institution. Specifically, the Bell Curve is used to ensure that the high value attributed to an 'A' is maintained because only a small percentage of students can be awarded it.
A bell curve appraisal method does not prove to be helpful for organizations with a lesser number of employees. You require more data to get the average of all the employees. An ideal range of the number of employees for the bell-curve appraisal would be at least more than 150 employees.
Many instructors grade on a curve for good reason. It allows them to see if they've created a test that isn't too easy or too difficult. It makes it easier to grade large classes and to grade more consistently across all of the instructor's classes.
Bell curves are useful for quickly visualizing a data set's mean, mode and median because when the distribution is normal, the mean, median and mode are all the same. The long tail refers to the part of the bell curve that stretches out in either direction.
The middle 80% under a bell curve (Figure 1) is the middle section of the bell curve that exlcudes the 10% of the area on the left and 10% of the area on the right. The 80th percentile (Figure 2) is the area of a left tail that excludes 20% of the area on the right.
On the bell curve, the Mean is in the middle, at the 50th percentile. The average or Mean score on most tests is 100 (Mean = 100). Most test scoring systems have a Mean of 100 and a Standard Deviation of ±15. Scores between 85 and 115 capture the middle two-thirds of the children tested.
To calculate the curved grade, multiply the original grade by 100, then divide by the maximum grade scored on the test.
A bell curve is a graph depicting the normal distribution, which has a shape reminiscent of a bell. The top of the curve shows the mean, mode, and median of the data collected. Its standard deviation depicts the bell curve's relative width around the mean.
A good example of a bell curve or normal distribution is the roll of two dice. The distribution is centered around the number seven and the probability decreases as you move away from the center. Here is the percent chance of the various outcomes when you roll two dice.
Disadvantages of Bell Curve
Using the bell curve model in performance management may be considered a rigid approach for rating employees. Sometimes managers need to put employees in specific gradients just for the sake of bell curve requirements. This happens more often when the teams are small.
Professors can move up the average score by a set amount. If the class average was 50%, for example, the professor might raise the average to 75%, effectively giving every student an extra 25 points out of 100. Similarly, if the average grade was a D, professors might change all D grades to C's.
High grade to 100%
Then the student with raw score 90%, gets a 10-point curve, but a student with a raw score of 60% gets a 7-point curve. A modification of this method is to compute the percentage of some other score (presumably ); Pros: I can't think of one. Cons: high-scoring students get a larger curve.
The equation for normalization is derived by initially deducting the minimum value from the variable to be normalized. Next, the minimum value subtracts from the maximum value, and the previous result is divided by the latter.
It's a method used when it is obvious that most of the students were unequipped to answer the question. While everyone will benefit from this type of curve, the drawback is that students won't really learn from this mistake unless the teacher re-teaches them and then adds a similar question in the next exam.
Grading on a curve places students on a normal distribution. This can sometimes feel unfair when a class is unusually high-performing, as some students might inevitably have their grades reduced after the bell curve calculation.
This method is no longer effective in ranking individual employee performance in modern group environments. A 360 feedback appraisal system instead of the bell curve method can effectively rank an employee's performance based on individual work, not against the work of their peers.
In statistics, an inverted bell curve is a term used loosely or metaphorically to refer to a bimodal distribution that falls to a trough between two peaks, rather than (as in a standard bell curve) rising to a single peak and then falling off on both sides.
Common examples of grade conversion are: A+ (97–100), A (93–96), A- (90–92), B+ (87–89), B (83–86), B- (80–82), C+ (77–79), C (73–76), C- (70–72), D+ (67–69), D (65–66), D- (below 65).