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Bell Curve: Definition & Impact on Grades

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  • 0:05 What Is the Bell Curve?
  • 1:17 How Grading on a Curve Works
  • 2:42 Standard Grading vs.…
  • 5:04 When to Use a Bell…
  • 6:32 Lesson Summary
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Lesson Transcript
Instructor: Jessica McCallister
You may have been graded on a curve in a high school or college class. This lesson explains the basic concepts behind the bell curve and grading on a curve, as well as their impact on grades.

What Is the Bell Curve?

Have you ever taken a high school or college class and were told on the first day that you were going to be graded on the curve? I'm sure you were left wondering, 'What does that mean? Will I get graded fairly?' These are often questions students ask when they are told they are going to be graded on the curve.

In this lesson, we'll explore what the term bell curve means and provide some examples of how it is used in grading systems. This is not a lesson in the statistical formulation of the bell curve; rather, it is a lesson to help you fully understand the impact on grades that the bell curve produces.

First, to help you better understand the basics behind the bell curve concept, let's start by explaining what the bell curve is. The bell curve is a statistical concept that is designed to establish a normal distribution. In a normal distribution, the data that is being distributed across the curved shape would be even. So, if you drew a normally distributed bell curve on a piece of paper and then cut it out, you would be able to fold it over perfectly. Its shape would be symmetrical. This is normal distribution, otherwise known as a normally distributed bell curve.

How Grading on a Curve Works

Grading on a curve impacts students' grades. There are different types of methods to grade on the curve, but here we'll discuss the most popular and classic method: the bell curve method. Imagine you were in class and your instructor said, 'Okay, this term you will be graded on the curve. The top 10% of students will receive an A, the second 10% will receive a B, the middle 60% of students will receive a C, and the bottom 20% of students will receive either a D or an F. It's up to you to work hard and try to earn one of those spots in the top 20%!'

If the grading is done that way for a class of 60 students, the grades will be distributed as follows: six students would get A's, six students would get B's, 36 students would get C's, six students would get D's, and six students would get F's.

Curved Grade Allotment (N = 60 students)
A's = 6
B's = 6
C's = 36
D's = 6
F's = 6

So the majority of students will be allowed to earn a C, and the grades, in this case, are normally distributed across a curve. On a bell curve like this, the top of the curve demonstrates the C's, the far right side of the curve is where the B's and A's are placed, and the far left side of the curve is where the D's and F's are placed. So, can you see which side of the curve you will want to be on in terms of receiving a grade? You want to be in the middle or right side of the curve!

A normally distributed bell curve
Normally distributed bell curve

Standard Grading vs. Bell Curve Grading Systems

Now, let's take a look at how this is different from standard grading. Let's say that in your class you were able to earn 1,000 points throughout the entire term. The points include tests, quizzes, papers, and class participation. In a standard grading system, you would receive a grade based on how many points you earned.

For example, if you did very well in your class and earned 980 points, you would earn a final course grade of an A. If you missed a few assignments and earned 800 points, you would earn a B and so on. In this system, it doesn't matter what someone else does, you are graded based upon whether you did the work and did it well or not. You are most likely familiar with standard grading.

Curved grading is different. Why? In a curved grading system, even if you earn 980 points out of 1,000, it doesn't necessarily mean you'll earn an A - it could mean you actually earn a B or even a C. Remember in our earlier example that only six students will be able to receive a grade of A and only six will get a B? How do you earn one of those grades?

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