Saturday, June 06, 2009

Thoughts on the Plus/Minus Grading Scheme

Before last Wednesday's Decemberists concert, I talked with my friend Patrick and some of his friends about how their education was going at that mortal and ancient enemy of mine, the University of Georgia.

One topic that came up--and it's come up in conversations with people from other schools, too--is the "plus/minus" grading scale. It's my opinion that the plus/minus system is only moderately beneficial in some cases, but that benefit is far outweighed by prohibitive flaws at both extremes of the scale.

Let's take a student, Steven. He's a good student who does well most of the time, never outright excelling, but never failing either. In a recent semester, Steven took four classes, and he got a B in each one. In two of the classes, he was right in the meat of the "B" part of the curve. In the third, he never really understood one of the units of the class, and he was lucky to scrape by with a B. And in the fourth, he really got into the class and participated a lot, but based on his test scores, the professor couldn't in good conscience give him an A.

Assuming these four classes are equally weighted toward his GPA, he's got a 3.0. Now, what if the plus/minus system were introduced?

The first two classes, Steven would still have B's. In the third, the professor wouldn't want to give him a C, but seeing his poor performance on one third of the class, he feels completely justified in giving a B-. And in the last class, though the professor couldn't justify an A, Steven's enthusiasm might push him up to a B+. So, with the additional assumption that the "plus" is as "positive" as the "minus" is "negative", Steven's GPA is still 3.0.

A supporter of the system would argue that this is exactly why the system is good. There's no net movement up or down, and it gives an outside observer an understanding of Steven's accomplishments that's both more precise and more accurate than the standard A/B/C/D/F system.

But what about shifting all those grades up one letter? Take Leon, who really does excel in most all of his classes. He also took four classes last semester. Leon rode the top of the curve in two of them, getting a solid A in each. He fell a little behind in this third, and after a rough final performance, was lucky to keep his perfect semester intact. In the fourth class, though, Leon didn't merely ride the top of the curve--he was "that guy" that ruined it for the rest of the class.

Obviously, on the standard scale, Leon has himself a 4.0. What about with plus/minus? First two classes, Leon's secure with his A's. In the third class, though, the professor might punish the less-than-stellar finals performance with an A-. But what about the fourth? No matter how much the professor might want to give our man Leon an A+, the A+ does not exist in most conceptions of the plus/minus system.

Steven's grades were normally distributed about "B", so under either grading system, his GPA was 3.0. But even though Leon's grades are normally distributed about "A", when pluses and minuses are added, his GPA drops to 3.925. Therefore, for students at the top of the grade scale, the net effect on GPA is neutral at best and negative at worst.

Now let's take the grades in the other direction. Ronald hasn't been doing particularly well--he got C's in his four classes. We'll say those C's were in the same manner as Steven's B's and Leon's A's--that is, two "middle C's", one "high C", and one "low C/borderline D". His GPA is 2.0 under the standard system. And, because Ronald's grades are normally distributed about "C", and because C+ and C- both exist, his GPA is still 2.0 under plus/minus.

But the fact that C- represents a 1.7 might raise some issues. For instance, what if that C- were in a major class? Most majors require a certain GPA in their major in addition to a certain overall GPA. That 1.7 could have detrimental effects on the major GPA a lot more than a 2.0 could--particularly because 2.0 is a favorite cutoff mark for "passing" versus "not".

For most students, plus/minus doesn't hurt or help more than a tiny fraction of a point. However, it does have the potential to significantly harm GPA of students at the top of the scale (because of the lack of an A+) and students on the brink of passing (because the C- hurts significantly more than the C+ helps).

Is there another argument in favor of plus/minus that I'm missing? Does anyone think it's justified despite these weaknesses?

Currently listening: Handel on the Law (in handy podcast form)

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