Statistics work only if I have a reference point

[trinityvixen]

So I did poorly on a chemistry test this week. I'm not fussed about it for a number of reasons--I've done well on other tests, this is a course I'm not sure I need, I've already taken it so I'm not concerned about knowledge I don't have. What I am is confused. I figured I'd throw this out to the people I know who know stats better than I ever have, do, or will.

Our grades are computed based on "z-scores," as befits a curve (apparently) that takes into account the fact we can drop one grade. This score equals [my score]-[test average]/standard deviation. The average z-scores of the class are then ranked. This is all well and good, but what rank/score equals what grade is not specifically ever stated. They say they chart the z-score averages, presumably on a histogram, and wherever there is a "break" in the graph--meaning a significant drop in the numbers of people at a given average z-score--there will be a change in letter grade. But since you have no idea where those breaks will be until all the tests are counted, including the final...what the hell will your grade be?

This is a useless way of keeping track of how well I'm doing. Also, pardon me and my non-statistical thinking here, but how does grading people based on how much better/worse they are than their peers accurately value what they've learned? This is not a new problem I know--this is the curse of the curve--but still!

Comments

[kent-allard-jr.livejournal.com]

It's always hard to distinguish (a) things one doesn't understand from (b) things that don't make sense. This definitely sounds like (b).

Our grades are computed based on "z-scores," as befits a curve (apparently) that takes into account the fact we can drop one grade.

Does he mean that dropping one grade makes scores normally distributed? Because it's simply not true: I did the same in my classes, and the grades were never normal (they were strongly left-skewed).

They say they chart the z-score averages, presumably on a histogram, and wherever there is a "break" in the graph--meaning a significant drop in the numbers of people at a given average z-score--there will be a change in letter grade.

So lemme get this straight ... In Class I, half the students get -1s and the other half get +1s, so he'd hand out Bs to the first half, and As to the other. In Class II, a third get -1, a third get 0, and a third get +1s, so he'd give the first third Cs, the second Bs and the third As? That's nuts.

But since you have no idea where those breaks will be until all the tests are counted, including the final...what the hell will your grade be?

So here's what they do: They take your scores, divide them by 5, take the square root, convert to their natural log, add 16, multiply it by your social security number and then ... guess! Great fucking system they have there!

[linaerys.livejournal.com]

I think a curve only works/is necessary if the test is much harder than it should be. I took a Computer Science class in college, and on the first test, the average was 15%. People who got 30% got an A. The professor apologized for making such a hard test.

Of course, as the person who got 95%, I was pretty annoyed, but honestly, that is the fair way. He shouldn't fail the entire class if he wasn't able to teach or write a test that accurately reflected his teaching.

[ivy03.livejournal.com]

That's how grades were done in the chemistry department. I know for certain it was how honors were determined. (Curse Hannah and her outlierness! Making suma unreachable for the rest of us!)

But if you think about what a grade is supposed to be, that is, a way of sorting people into categories of how well they've done, then this is the most logical way to do it. You make the grades break where the clusters break, so you aren't punishing people or rewarding them for being one point over or below an arbitrary pre-set line. If the point of grades are to group students into groups relative to each other, this method makes sense.

If the point of a grade is to rate a student against an outside benchmark irrespective of how other people are doing, then obviously this doesn't make sense.

But you know, I majored in a department with the lowest average GPA at Princeton. I was told in one class that only one student would be receiving an A. So my experience is not really a reassuring one with respect to grades.

[xannoside.livejournal.com]

Reasons to grade on a curve:

1) Assigning grades based on the 100-pt scale divided by the # of questions is not reflective of student ability.
2) The school administration wants less Cs and Ds.

This just sounds like pretending to be awesome at stats.

Plus, if you have 40 people or less, you won't have a normal curve. And if they have less than 100, they should be using T-scores, not Z-scores, anyways.