Friday, February 02, 2007

Homework 1 graded

I finished grading Homework #1. The average was 80% (56/70) with a max of 90% and a standard deviation of about 7%. Pretty good.

I put solutions up on the web page. Note that they are not necessarily maximally simple; for example, I think Amitabh had a nicer proof for #1 than I did, and Moritz had a nicer proof for #3b then I did. Also almost everybody did #2 by induction, which is about equally easy.

Most common mistakes were: #1 -- using Pr[A and B] = Pr[A] Pr[B]; #3a -- messing up the signs; #5 -- failing to do if *and* only if.

Re #3a -- I realized that I kind of messed up the signs in the problem statement. Actually, what I wrote was technically correct, but misleading ;) Anyway, it only cost people -1 out of 70.

Re #7b -- The hint I gave on the blog was actually too complicated. I simplified problem #7b at the last minute, and the hint was needed for the original more complicated version.

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