U.S. grading system: Why discrete grades? Background

In the U.S., many colleges adopt an A–F grading scheme, topped with pluses and minuses.

Professors vary in the way they assign A to F grades, but my sense is most rely on discrete “bins”. For example, some professors use relative grading and (roughly) give an A to the top third or the class, a B to the second third, etc. Others will give an A to students who get between, say, 100% and 95%, and A− to students between 94% and 90%, a B+ to students between 86% and 89%, etc.

Using discrete bins necessarily implies some knife-edge cases where a student is right at the border with the next bin, and it would have taken very little for that student to jump into that next bin and improve their letter grade by one “level”.

That would be all fine (I guess) if letters where only honorary titles, and what mattered was the actual underlying percentage grade. However, in most places (i.e., all the places I know), it is the letter grade – and never the percentage grade itself – that is attached to the student’s academic record.

This could be an issue per se if a student wanted to show proficiency in a particular topic and got, for example, an A− that was in fact very close to an A. That student’s academic record will forever show very good but not perfect for that course, whereas it should really indicate almost perfect. This creates understandable frustrations from students who nearly made it.

Things get even worst when averaging through different courses and computing one’s GPA, since most colleges then use a scheme like the following one: 


A: 4.00
A−: 3.67
B+: 3.33
B: 3.00
B−: 2.67
C+: 2.33 
…


Again, this means that the half of a percent a student needed to jump from an A− to A will have a significant effect on that student's GPA, where really, the effect of getting one more half of a percent should be negligible, regardless of the baseline percentage one is starting from.

In a world where GPAs are taken so seriously, this means that half of a percent can make a significant difference in a student’s life and career. Again, this can create understandable frustrations for students who nearly made it.

My question

I know that there are “palliatives” to deal with the discrete grading scale and avoid some of these knife-edge cases, such as rounding up decimals. In all fairness, I don’t think those are real solutions (rounding up decimals just moves the problem from one threshold to another), but that's not what I am interested in here.

What I would like to know is:


Are there any arguments in favor of using a discrete grading scheme like the one above?
Or is just a product of history and the difficulty to coordinate practices across the vast number of U.S. colleges?


In other words, is anyone claiming that the discrete scale has virtues of its own other than the fact that it is used in so many places – creating comparability –, and that it would be hard to coordinate a change at all those places at once?
I am in favor of using the US discrete grade system. Some thoughts I will address are as follows:

As the OP mentions, in the US, the ABC grade system is relatively standardized and well understood by a large variety of institutions. It makes comparisons among students from a variety of schools more compatible (although, yes, not perfectly comparable). 

Beyond this, I feel that the ABC system delivers the appropriate amount of granularity. After all, you have to truncate grade percentages somewhere. In some sense every system is discrete. 

One method that I have used for my students in math classes in the US is a clustering system guided by the grade cutoffs. The percentages are guarantees. After that, I cluster students into grade groups, with students who barely missed the cutoff still usually getting adjusted up. For example, let's say that we have the following percentages:


  93.23%, 93.17%, 92.88%, 92.81%, 92.08%


I would give the first four students A's and the final student an A- under normal circumstances. (It is usually more nuanced than that). I feel that the grades I gave my students usually very accurately described their proficiency relative to the stated outcomes of the course.  This was also vastly true when I was a student. In fact there were times where I would prefer to take my A grade than my raw percentage. An 'A' sounds much better than "I got 83% raw, but I was the second highest grade in the course."

In my opinion, the ABC system actually is a stronger system than a raw percentage system. Is a 95% in Dr. Haskin's Calculus I class better than a 97% in Dr. Bodrel's Calculus I course? Both would result in an A grade. Or how would a 97% in Calculus I compare to a 97% in Floral Design? By using the ABC system, we effectively run a sort of "low-pass" filter on the grades. In my opinion this actually allows for a fairer comparison between students. 

Let me also add that having a raw percentage system would be awful from the perspective of an instructor. Students already anguish about their grades. Now picture if they had to anguish about not just getting an A, but getting 100%. You would end up having students with 98% in the course worrying about getting 99%. It just becomes excessively fiddly. As a former student myself, I would actually hate a raw system. I was grateful for being able to get an A with 94% in a class without having to worry about pushing it up to a 95%. 

I will close by saying that school and grades are not always fair. Such is life. No matter what system you use, there will be drawbacks from one perspective or another. There is no system that is perfect. 



Addendum: Several comments have mentioned thoughts on being graded relative to others. Note first that my proposed method never moves a student down a grade, only up. Several comments seem to overlook that. 


  @BartoszKP: "You are grading not 'their proficiency relative to the stated outcomes,' but 'their proficiency relative to the stated outcomes and relative to other students.' Did other students doing well make this student suddenly know less?" 


How could raising a grade indicate a student "knew less"? It seems that raising a grade would actually indicate a student knew more. 

I guess I could grade students solely on their raw scores, 100ths of a percent be damned. Then students would be graded with less relativity to their peers. But wasn't such a "zero-tolerance of rounding up" system one of the named drawbacks of the ABC system? It seems like we are trying to play both ends of the flute here. I can see that it would be patently unfair if I rounded students down a grade. I must say though, I have never had a student complain about being graded "relative" to their peers when I rounded their grade slightly up. 

Secondly, to continue the thoughts on being graded relative to one's peers, we need to keep in mind that it is next to impossible to provide students from current semesters the identical exam experience as students from previous semesters encountered. By adjusting scores within semester, this can help control for slight variations in exam difficulty from semester to semester. Note that I do not always have complete control over exams, since they often must be written on teams with other professors. All told, which system seems more "fair," a system that does not control at all for variability in exam writing styles or a system that attempts to account for that?