Basically, this tells me you perceived questions 2 and 4 as far more difficult than the others. This was borne out by the average score for number 2. It is a difficult problem, mainly because it seems like you should be able to solve it by energy conservation, but you can't. Number 4 on the other hand was not so difficult as it seemed - those who tried it did quite well. Compare this to the poll at right; most of you thought question 3 was the most difficult, but you did well.
The second set of plots is just a histogram, with the second plot having more narrow bins. Broadly, it is very good - the average was much higher than I expected, and there are not a lot of people on the left-hand 'tail' of the distribution.
Looking at the finer-grained plot doesn't tell me much more, except that the large majority of you have no cause for worry - you should consider 75 or above on this exam to be a good score, it was very tough. Those of you that were at 90 and above did truly outstanding work, and I was quite impressed.Those of you on the lower side have absolutely no cause for worry yet. No one has below a C average in the course now. This exam is only 10% of your grade; homework is worth 12.5%, the final is worth 25% of your grade. If you scored (say) 20 points lower than you would have liked to, this is only a (roughly) 2 point change in your grade - barely enough to change a + to a -. The message is: keep at the homework diligently, and work hard at the 3rd exam and the final.
(By the way: there is an 80% correlation between homework grades and overall grades ... the highest of any segment of your grade. Meaning, I like to think, there is some sense in slogging through the homework each week.)
(Also: the last poll option seems to roughly correlate with the tail of the distribution, which makes sense to me. At least one of you should have picked the first option though.)
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