“At the end of the semester, the total scores for all students will be arranged in numerical order, the score that corresponds to the 99th percentile (S99) will be determined, and then letter grades will be
assigned based on this percentile score as follows:
A: Total Score ≥ 0.90 x S99
B: 0.80 x S99 ≤ Total Score < 0.90 x S99
C: 0.70 x S99 ≤ Total Score < 0.80 x S99
D: 0.60 x S99 ≤ Total Score < 0.70 x S99
F: Total Score < 0.60 x S99 or if you fail to complete 10 of the 12 lab
projects”
This is the explanation the department of chemistry for my college gives. But I don’t understand, so please explain it to me like I’m five.
In: Mathematics
The 99th percentile is the value that is greater than 99% of the values in the set. So, if there are 100 students, the 99th percentile is the score that 99 students did worse than and 1 student did better than.
In this grading system everyone’s performance is judged relative to one person’s grade.
The person they choose is *almost* the highest scoring person. They skip the highest few people in case those people did much better than the rest of the class (e.g. because they have a tutor that classmates had no access to). For every 100 people in the class they skip one person at the top of the class when looking for the person to grade everyone against.
A popular next step that many courses would employ next is to ask what number needs to be added to that person’s grade for them to get a 100. This curve takes a similar step but instead asks what number needs to be multiplied by that person’s score to get to 100. For example, if the chosen person got an 88 then we’d choose 1.1363636… as the number, since 88 * 1.363636… = 100.
The grader then multiplies everyone’s raw score by that number and sorts into buckets. If you land in 90-100 then you get an A; 80-90 is a B, and so on.
(Note that the description you posted does the math in a different order, but this is equivalent).
In college, we had a thing called “curve busters”. The idea is that if a test is graded on a curve, one genius in the class can ruin it for everybody. Everybody gets mad at the curve buster in the class.
So to make it fair, when you’re calculating the curve, you ignore the very best 1% of the class. Whatever the top score in the remaining 99% was, that’s the score used to calculate the curve.
It looks like the grades are then calculated as a percentage of what that top score was. If you do better than 90% of that top score, you get an A. 80% gets you a B, and so forth.
Now when I was in school, many of the TAs had a different method: grades on a test typically don’t fall on a bell curve as you might expect. Instead, there tends to be clusters. A smart grader will put the boundaries between grades between those clusters. This reduces the number of students who come and complain about their grades that were just short of the boundary to the next letter grade.
They’re setting the top score to 100 and then calculating everybody’s scores relative to that.
Think of it this way. You might have 12 labs, and each would have 100 points available. The total amount of potential points is 1200. However, they don’t expect everyone to get everything. Instead, they’re going to drop the “out of 1200” part of the grade.
The 1200 will be the 99th percentile of obtained marks, meaning so high that only 1% of students get a perfect. So let’s say out of 200 students, you have 1 perfect, 1 person at 1150, and then two people each with 1100. The represent the 99th percentile 1% of the class, or 2 people, got higher). So now you’re being graded out of 1100 instead of out of 1200.
Rather than assigning grades based on how many points you earned out of the available points, grades are assigned based on how you rank against the rest of your class. The people with the highest 1% of points earned determines what is considered the “maximum available points”. Grading is then done as usual, just using this adjusted maximum rather than the actual maximum.
The idea is that, if the assignments were so hard that *no one* got a perfect score, then the points needed to qualify for a particular grade are shifted down to compensate. This prevents things like flunking the whole class because no one got over 59% of the points that were available. If a situation like that happens, it’s more likely the fault of the instructor than the students, so this system adjusts the grading to favor the students.
This seems like a weird version of curved grading. Basically, the cut points for grades depend on what the 99th percentile is- to make this easy, if there are 100 people in the class, the 99th percentile is highest grade. If there are 300 people, the 99th percentile is the average of the top three grades.
Cut points are determined from this grade. For example, if the 99th percentile is a 100, then an A= 100 *0.9= 90%, a B=100 *0.8=80%, and so on.
If the 99th percentile is a 93, then an A=93 * 0.9=83.7, a B=93 * 0.8=74.4, and so on.
Realistically, in a college class, there’s probably at least one or two people who will get 98-100 percent on everything, so it’s likely the grading scale will be the common intervals, but in case the class is super difficult, this will adjust everyone’s grades up a bit.
You are in college and don’t understand percentile? Or the motivation for using percentiles in grading? Take 100 random humans and measure their height. Arrange them from shortest to tallest. The guy at the end (the tallest one) is in the 99th percentile – he is taller than 99 other guys. Him and the 9 guys preceding him are in the 90th percentile: they are all taller than 90% of the sample (remaining 90 guys) etc etc etc. If you want to grade them, you define grades A: 99>x>=90th percentile; B: 60>x>=89 and so on.
Slightly off topic rant, someone gored my Ox :
they replies i’ve seen here are all pretty much spot on. On the other hand there is a problem with “grading on the curve” if you are to strict with it. a class i took in college was pretty easy, at the end of the semester all students had averages between 92% and 100% the professor proceeded to give the proper grades on the bell curve, meaning the 92-93% scores were failing, 94% was a D, 95 a C 96 -97 a B, 98 an A, (the numbers might be off a little, this was 40+ years ago) there was a great wailing and gnashing of teeth several meetings with department heads and deans, everyone ended with an A for the course. and the professor was much more reasonable in the future.
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