How to curve grades

Grading on a curve means adjusting scores after a test so the results fit a target, usually because the test turned out harder or easier than intended. There are two common methods, and they make different trade-offs. This guide sets out both with worked numbers, then the mistakes that make a curve unfair.

Decide whether to curve at all

A curve fixes a problem with the test more than with the students. Before you adjust every score, look at each question. If one question has a wrong answer key, or most of the class got it wrong for a reason you can name, the fairest fix is usually to remove that question or give everyone its marks, then recalculate. A curve moves every score, including the ones that were already right.

Method 1: the additive curve

An additive curve adds the same number of points to every score. The number is the target minus the highest raw score. If the highest score is 85 and the target is 100, add 15 to every score. A raw 62 becomes 77, and a raw 70 becomes 85. Stop any score at 100 so nothing goes above the maximum.

The method is easy to explain, and it keeps every gap between students the same. Its weakness is that it lifts a student who scored 10 by exactly as much as one who scored 80, which some teachers find too generous at the bottom of the scale.

Method 2: the scaled curve

A scaled curve multiplies every score by the same factor: the target divided by the highest raw score. With the highest score at 85, the factor is 100 divided by 85, about 1.1765. A raw 62 becomes 72.94, a raw 70 becomes 82.35, and a raw 85 becomes 100. A raw 0 stays 0.

A scaled curve keeps zero at zero and gives more points to higher scores. It also widens gaps between students, so a small difference in raw marks becomes a larger one in the curved marks. That can matter near a grade boundary.

Both methods can be checked with the grading curve calculator, which shows every score before and after the curve.

Mistakes that make a curve unfair

The first mistake is anchoring on one score. The highest raw score drives both methods, so a single student who scored unusually high sets the curve for everyone. If that score is an outlier, look at the distribution, or agree a target that does not depend on one result.

The second is curving after you have seen the marks and then choosing the method that gives a result you like. Decide the method and the target before you calculate, and write them down.

The third is curving scores that have already been rounded to a grade letter. Curve the numeric marks, then assign letters, so each student’s position is still measured on the same scale.

How Votense handles grades

Votense does not apply a curve. Exam mode grades each attempt against a pass mark and grade bands that you set, and you can override any automatic mark on the Grade screen. Manual marks replace the automatic ones for that answer. To use a curve, calculate the curved numbers, then set the bands or enter the marks that match them.

You can read more about how quizzes and exams are scored in the quizzes guide, or make a graded test with the exam maker.