What is the bell curve system?

The bell curve system is among the terms that university students often encounter. In this system, in which the general definition of the class average is calculated and the passing grade is determined, the grade calculation method is shaped accordingly.

So, what is the bell curve system and how to calculate grades? Here is some information on that subject.

The Bell Curve system can have an advantageous position for students among the grade calculation methods.

With the bell curve method, the failure rate is somewhat reduced. This system can also provide a disadvantage depending on the success situation in the classroom. So, what is the Bell Curve System and how to calculate grades? Here is some information on that subject.


The main purpose of using this method is to grade students taking a certain course according to their performance in the group taking that course, not based on their absolute grades over 100 points. Performance is where a student is in relation to the class average. In this system, the mean and standard deviation of the class are of great importance.

When calculating grades;

Grades you get with the formula (HBN* – HBN average of the class) / Standard deviation) will be converted to Z Standard Grade and Z standard grades will be converted to T Grade with the formula (T=10Z+50). The calculated T Grade (TSkor) will be your new grade.

(*HBN=Raw Success Score=Your midterm and final weighted average) The letter success grade of each student according to their T-Score is determined with the help of TABLE-3 below, taking into account the T-Score limit values ​​corresponding to the grade level.

If a student’s grade is equal to the average grade of the class, their T-Score will be equal to 50. For example, if the class average is 69 and the student’s grade is 69, the letter grade is CB, which corresponds to 50 T-Scores in that grade range (class level); but if the class average is 52 and the student’s grade is 52, the letter grade corresponding to 50 T-Scores in that grade range (class level) is CC.

To give another example;

  • Class average: 69
  • Standard deviation: 9
  • Your Exam Average: 50
  • According to this assumption (50-69)/9)=-2.111 then (10×-2.111)+50=28.89
  • Your new score has been determined as 28.89.


Let’s look at the letter grade we got from the table below according to the assumptions we made;

Since our class average is 69, we look at the “Very Good” group in TABLE-3. Since our T Grade calculated according to the class average and standard deviation in the Relative Evaluation System is 28.89, our letter grade is determined as FF.

As can be seen, a student under these conditions fails to get a passing grade in the relevant course despite fulfilling the lower limit of 50 points in the Final and Raw Success Grade (Weighted Average)! As we mentioned at the beginning, the class mean and standard deviation are very important. So, according to the formula, there are two factors that affect the student’s T-Score; the distance of the student’s grade from the average grade and the standard deviation of grades in the class.

As the student’s grade rises above the class average, the T-Score will increase, and vice versa, it will decrease. If our grade is below the class average (HBN- HBN average of the class)/Standard deviation=Z, the result will be negative.

Therefore, it will have a negative effect on our new score (TSkor) in the formula T=10Z+50. In addition, if the standard deviation is too large, the Z value may be small. This directly affects the T value, that is, your grade, and leads to a low letter grade. And also where the standard deviation is low, deviations from the mean will have a greater effect on the T-Score.

Source: https://www.hurriyet.com.tr/
Photo: https://www.investopedia.com/

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