Journals

I was really pleased with the journals. Two were short of three full pages, please watch this. But the journals this week were really thoughtful and had some good ideas. Keep it up!

HCOL 195 Class Members, Post Your Comments/Questions

OK, folks, the blog is started.

You can click on the comments section of any blog entry and log in under your Google account (if you do not have one, you can sign up for one here), and submit comments and/or questions about any item.

I will check the blog page on a regular basis and respond to questions and comments; others are welcome and encouraged do the same.

Please note that entries prior to September 1 are for a graduate class I taught a year ago, and have no relevance to our class. You are welcome to read them and comment on them, but they are quite advanced!

Bill

Monty Hall Problem

The basic thing to keep in mind is that initially, your chance of choosing the door with the prize is 1 in 3. This does not change when Monty shows you (as he must, under the rules) that one of the remaining doors has a goat. You knew that already, you just didn't know which door. So, if your door doesn't have the prize, then the one he didn't reveal must have the prize. Since there is a 1 in 3 chance that your door has the prize, there must be a 2 in 3 chance that the one he didn't open has the prize, and it pays to switch.

One way to think about it is to suppose that there are 100 doors, one of which has the prize. The chance that you initially choose the door with the prize is only 1 in 100. So, there is a 99 in 100 chance that one of those other doors has it. Monty knows which one it is, so he can open 98 doors and never reveal the prize. He will do this in 99 out of 100 games played, and in those 99 out of 100 games, the door he doesn't open will have the prize. In the 1 out of 100 games played where you chose the prize door, he'll open 99 doors at random, and the remaining door will have a goat. So in 99 out of 100 games played, it pays to switch.

I left the class with several variations of the problem to think about for class on Friday:

1. Ignorant Monty: Monty doesn't know where the prize is, and sometimes randomly opens the door with the prize. Is it an advantage to switch, to stay, or doesn't it matter?
   
2. Angelic Monty: Monty knows where the prize is. If you choose the door with the prize, he opens it and congratulates you. If you choose the wrong door, he opens a door without the prize and offers you the chance to switch. Is it an advantage to switch, to stay, or doesn't it matter?
3. Monty From Hell: Monty knows where the prize is. If you choose the door with the prize, he opens a door without the prize and offers you the chance to switch; but if you choose a door with the goat, he opens it and says, "too bad, you lose." Is it an advantage to switch, to stay, or doesn't it matter?
4. I didn't mention this, but we'll think about it as well. This is "Mixture Monty." Before coming on stage, Monty flips a coin. If it comes up heads, he behaves as Angelic Monty on stage. If it comes up tails, he behaves as Monty From Hell. Is it an advantage to switch, to stay, or doesn't it matter?

Comments on problem sets

These are the main rules I suggested for handling the problem sets.

1. Unless otherwise stated, all problem sets are to be done together in your study groups. I expect one paper to be turned in for each study group. If there are disagreements within the group, spell them out in that paper.
   
2. Be sure to do every part of every question.
3. Be sure to explain carefully how you got each answer. The reasoning process is crucial.
4. Do reality checks. Do the results seem reasonable? If they don't seem reasonable, say so, even if you don't know why!
   
5. Carry units and cancel them to make sure that the units of the answer are correct. In particular, be careful to distinguish length, area, and volume.
6. Use metric system and powers-of-ten (scientific) notation, it's much more error resistant.
   

September 11, 2008 10:13 AM

Mini-Symposium on Statistical Consulting

There was a mini-symposium on statistical consulting at Columbia University yesterday. Andrew Gelman has posted some interesting things from the symposium, and promises more. Note in particular Rindskopf's Rules for Statistical Consulting.

I do recommend reading Andrew's blog on a regular basis. You'll learn a lot.

New version of Missing and Censored Data pdf

I've put a new version of the Missing and Censored Data pdf on the website. It turns out that there was nothing wrong with the equations, but poor notation and arrangement of the equations made them difficult for me to interpret correctly while standing on my feet! I hope this version does better. Note in particular charts #16-17 and the charts on Example 7 beginning with Chart #21.

Software Carpentry

In class, I mentioned the Software Carpentry project.

Here is a link to a version of the course that was given at in 2005. There are both written and audio files.