What Is Grading on a Curve?

Grading on a Curve

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Grading on a curve has long been disputed in the academic world, just as weighting scores have, too. Some teachers use curves to grade exams, whereas other teachers prefer to assign grades with the percentages as is. So, what does it mean when your teacher tells you that he or she will be "grading on a curve"? Let's find out!

The Curve Basics

In general, "grading on a curve" is the term used for a variety of different methods of adjusting a test grade in some way. Most of the time, this type of grading boosts the students' grade by moving his or her actual percentage up a few notches or boosting the letter grade. Sometimes, though, this method of grading can be irritating to students because some kids' grades can be adjusted at a higher percentage than others depending on the method used to curve.

What's the "Curve"?

The "curve" referred to in the term is the "bell curve," which is used in statistics to show the distribution of any set of data. It's called a bell curve because once the data is plotted on a graph, the line created usually forms the shape of a bell or hill. In a normal distribution, most of the data will be near the middle, or the mean, with very few figures on the outside of the bell, the extreme outliers.

Why Do Teachers Use a Curve?

Curves are very useful tools. They can help a teacher analyze and adjust scoring if necessary. If, for example, a teacher looks at her class's scores and sees that the mean (average) grade of her midterm was approximately a C, and slightly fewer students earned Bs and Ds and even fewer students earned As and Fs, then she could conclude that the test was a good design if she uses a C (70%) as the average grade. If, on the other hand, she plots the test grades and sees that the average grade was a 60%, with no grades above an 80% then she could conclude that the test may have been too difficult.

How Do Teachers Grade On a Curve?

There are some ways to grade on a curve, many of which are mathematically complex (as in, well beyond SAT math skills required). However, here are a few of the most popular ways that teachers curve grades along with each method's most basic explanations:

Add Points: A teacher tops off each student's grade with the same number of points.

  • When It Can Be Used: After the test, a teacher determines that most of the kids got questions 5 and 9 incorrect. She may decide to add the point that each question was worth to everyone's score.
  • Benefits: Everyone gets a better grade.
  • Drawbacks: Kids do not learn from the question unless the teacher offers a revision.

Bump a Grade to 100%: A teacher moves one kid's score to 100% and adds the same number of points used to get that kid to 100 to everyone else's score.

  • When It Can Be Used: If no one in the class gets a 100%, and the closest score is an 88%, for example, a teacher could determine that the test was too tough. If so, he could add 12 percentage points to that kid's score to make it 100% and add 12 percentage points to everyone else's grade, too.
  • Benefits: Everyone gets a better score.
  • Drawbacks: The kids with the lowest grades benefit the least (a 22% plus 12 points is still a failing grade).

Use the Square Root: A teacher takes the square root of the test percentage and makes it the new grade.

  • When It Can Be Used: The teacher believes everyone needs a little bit of a boost but has a wide distribution of grades (a few kids have A's, etc.) So, she takes the square root of everyone's percentage grade and uses it as the new grade: √x = adjusted grade. Real grade = .90 (90%) Adjusted grade = √.90 = .95 (95%).
  • Benefits: Everyone gets a better score.
  • Drawbacks: Not everyone's grade is adjusted equally. Someone who scores a 60% would get a new grade of 77%, which is a 17-point bump. The kid scoring the 90% only gets a 5-point bump.

Who Threw Off The Curve?

Kids in class always get annoyed with that one student who messed up the curve. So, what does that mean, and how did he or she do it? Above, I mentioned, "extreme outliers," which are those numbers at the very ends of the bell curve on a graph. In class, those extreme outliers represent student's grades, and they are responsible for throwing off the curve. For example, if the majority of testers earned a 70% and only one student in the whole class earned an A, a 98%, then when the teacher goes to adjust the grades, that extreme outlier could mess with the numbers. Here's how using the three methods of curved grading from above:

  • If the teacher wants to add points for missed questions to everyone's grade, but the highest grade is a 98%, then he or she can't add more than 2 points because it would give that kid a number above 100%. Unless the teacher is willing to give extra credit for the test, then he or she can't adjust the scores enough to count very much. Kids who scored a 67% would be annoyed with this.
  • If the teacher wants to bump a grade to 100%, everyone will again only get 2 points added to their grade, which isn't a significant jump.
  • If the teacher wants to use the square root, it isn't fair to that student with the 98% because the grade would only go up one point, and the student or student's parents may complain that kids with lower scores got a better boost.