Real Life Math
June 5, 2011 9:46 PM   Subscribe

Real Life Math: How could raising everyone's grade on one quiz result in some people getting a lower grade average for all the quizzes for some students?

Some background: I'm in class this quarter where we have these terrible pop quizzes that make up 15% of our grade. They're done using Turning Point Technology (not really important to the question, but does explain how the error was made, later) and the professor posts the grades online.

There have been 7 quizzes throughout the quarter, and the lowest one was dropped. The last quiz we had, the 7th one, there was an error in the grading and every answer for one of the questions was marked incorrect. Everyone in the class got that one wrong. The professor hadn't noticed when she first put up quiz grades, however--she posted on the website every quiz and then an average (which had the lowest score dropped).

An hour later, she sent out an e-mail, notifying us of the error, and that the grades were updated. She added, "This means that almost everyone has a slightly higher score as their overall grade, but a few people dropped by some points." In addition, the overall average (which the professor posted to the website) also went up by a few points.

I should totally be studying for the final for this class, but I keep getting caught up wondering how someone could get more points on a quiz, yet have their overall average drop.
posted by Ideal Impulse to Grab Bag (5 answers total) 1 user marked this as a favorite
Response by poster: Sorry, by "incorrect," I mean every answer was considered incorrect, not that the machine mixed up which one was right and which ones were wrong. So if someone answered the wrong answer for that question, they would originally gotten that one wrong, as well.
posted by Ideal Impulse at 9:57 PM on June 5, 2011

As you've described it, you're right. There is no way anyone's grade could have decreased if everyone was marked incorrect originally, and now some people are marked correct. It would also be impossible to lower any grades by throwing out the question entirely, or by throwing it out only for some people.

The only way someone's grade could decrease is what hal_c_on said--if their incorrect answer was wrongly marked correct initially. Either some people were marked correct before, or the professor's explanation is wrong. (Maybe they're just used to giving that explanation, because most of the time when they correct grading errors, some people are wrongly marked correct?)
posted by equalpants at 10:14 PM on June 5, 2011

Best answer: Are the quizzes curved?
posted by jz at 10:43 PM on June 5, 2011 [1 favorite]

crap, good point, jz. OP, please mentally prepend "assuming the grades are not curved" to my answer...
posted by equalpants at 10:46 PM on June 5, 2011

Response by poster: You got it, jz! I talked to someone else in my class and figured it out: the final, average grade we got was curved. I hadn't noticed it before because--braggin'!--my curved score and real average were the same.

So if the score is curved and the average went up, then some people's z-scores (for lack of a better term; I don't think professor created the curve based on standard deviations) went down.
posted by Ideal Impulse at 10:53 PM on June 5, 2011

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