# How to Calculate Class Interval

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Grades, like as not, are an important aspect of most forms of structured learning. Inevitably, teachers and professors have to rely on some kind of objective system to assess the performance of individual students. Since the range of performances in any class of significant size tends be be wide regardless of subject, both on individual assignments and end-of-term final grades.

Most teachers assign a certain low percentage of very high and very low grades, a modest percentage of "okay" grades between the extremes and the class average, and a large cluster around the class average. In the U.S. system, these grades usually range from A to F. But numerical scores vary wildly across academic situations. How can graders account for this?

Grading on a curve and dividing the student performances into class intervals based on time-tested statistical criteria helps make this conversion a standard process and can iron out some of the effects of overly difficult or easy exams and other unwanted situations.

A typical grade percentage chart in the United States will show letter grades ranging from F to A in order of improving performance, typically with the grades other than F given sub-gradations, e.g., B+ and C−. "E" is skipped; F is a failing grade and thus does not require further scoring.

An alternative (and sometimes complementary) system involves GPA, or grade-point average. This ranges normally from 0.00 to 4.00, with each interval number corresponding to a letter grade. That is, 0.00 is F, 1.00 is D, 2.00 is C, 3.00 is B and 4.00 is A. Getting to "+" and "−" gradations requires moving up and down in increments of 0.33.

Both of these systems map onto percentage systems in a fairly consistent way. The range of 1.67 to 2.33 is the range of C grades, and also corresponds to percentile scores between 70.0 and 79.9. These percentile scores, however, are often scaled scores rather than raw scores because of the phenomenon of grading on a curve.