Q&A: How to Grade Partial Credit

Cross-posted from here.

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Question

When grading problems on quizzes and exams, I often break them down into sub-problems, each worth a portion of the total points. I use rubrics to award partial credit for each sub-problem. However, this practice leads to students arguing that their incorrect work deserves more partial credit than what I gave them. For example, a student might claim that their mistake only warrants a deduction of 0.1 points rather than the 0.3 points I deducted.

To avoid these disputes, I’m considering switching to a binary grading system where each sub-problem is graded as either completely correct (full points) or completely incorrect (zero points), with no partial credit awarded.

My questions are:

1. Do you award partial credit when grading, or do you use a binary grading system?
2. If you do award partial credit, how do you justify and explain the specific point deductions to students who argue that they deserve more partial credit?

Answer

In my experience, binary credit on sub-problems is the way to go. Once you open the door to interpretation on partial credit, you get your time burned by pointless debates (pun intended) with adversarial students.

But the thing is, you have to make sure that the sub-problems are

• sufficient in number, so that there is room to make a few mistakes on the quiz and still get a good grade, and
• sufficiently atomic, so that it feels reasonable to grade them on a binary scale.

The goal is for the grade calculation to be totally obvious and feel fair enough that the student doesn’t argue against its mechanics.

In this way, grade calculations are like laws: you want them to be both clear and fair. If you have unclear laws then people will waste your time in court arguing about interpretation. If you have unfair laws then you’ll hurt good people and people will rebel against you.

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