i don't think i quite buy that..

here is me and john discussing it via email than IM:

-------

not exactly, because 2/3 of the time, the prize will be behind one of

the two you didn't pick, and he's telling you that it's definitely not

behind one of them. if you always choose a, it'll be a 1 of every 3

times, but it will be b or c the other two times, and he's telling you

which one it's definitely not.

i found a simulation you can try..

http://147.4.150.5/~matsrc/MontyHall/MontyHallSim.html

click a door to choose it, and then it will show another door with a

piggy to denote that door sucks. you may then click the same door again

if you wanna keep it, or choose the remaining door. if you do it many

times (and i've done it a few just now), the average should be about 1/3

for keeping your original door, and 2/3 for switching.

-----Original Message-----

From: dan widrew [mailto:widrew@mediaone.net]

Sent: November 15, 2001 4:32 PM

To: John Lavallée

Subject: RE: i want a math problem named after me..

that's true, but you could also say that once he takes out the 3rd door,

you now know that the correct door is the one you have or hte other one.

so now it's 1/2 likely to be either one. since you're making a new

choice, you get a new probability of it being correct

> the idea is when you choose originally, it's 1/3 likely to be whatever

> you chose, and 2/3 likely to be one of the other doors. now, if one

> of the other two is taken out, then your choice is between the 1/3

> chance that you originally took, and the 2/3 chance of the combined

> other doors. 1/3 of the time, it will be the door you originally

> chose, but 2/3 of the time, it's one of the other two, and by removing

> one of them from contention, he is in essence allowing you to choose

> the 2/3 probability by taking the other door.

>

> -----Original Message-----

> From: dan widrew [mailto:widrew@mediaone.net]

> Sent: November 15, 2001 4:23 PM

> To: John Lavallée

> Subject: RE: i want a math problem named after me..

>

>

>

>

> hmmm that's weird. i dont know if i quite buy it. it seems to make

> sense that way, but the thing is.. after he takes 1 door out, then *no

> matter

> which* of the other two doors you'd originally chosen, you have a 2/3

> chance of the other door being correct. that's impossible, theyre

50/50.

> if you choose A and he shows C is bad, then it's 2/3 likely to be B,

but

> if you choose B and C is bad, it's 2/3 likely to be A?

-------------

King Nixon (5:32:24 PM): that math thing is neat

King Nixon (5:32:27 PM): that you sent me

tiiznit (5:37:22 PM): i explained the math thingie, yo uexplain that

King Nixon (5:37:34 PM): hm?

King Nixon (5:37:41 PM): ooh new email

tiiznit (5:37:52 PM): new email?

King Nixon (5:38:24 PM): that you sent to me, explaining the math thing

tiiznit (5:38:43 PM): ah

tiiznit (5:38:46 PM): did it make sense that time?

tiiznit (5:39:02 PM): it dint make sense to me at first either, until it clicked

King Nixon (5:41:23 PM): see, it does make sense waht youre saying. but i can explain it to be 1/2 and that makes just as much sense to me

tiiznit (5:42:29 PM): the problem is that it isn't strictly 1/2.. it's 1/2 between your original choice and whatever is left of B and C.. and that one of them was removed isn't strictly arbitrary, it was because it is a zonk. so the one left over has a higher probability of not being a zonk

tiiznit (5:43:31 PM): (well, it is arbitary in the 1/3 of the time that A was right in the first place, but the other 2/3 of the time, it's whatever of B and C wasn't taken out)

King Nixon (5:43:31 PM): if you look at it as either keeping the choice you made or switching, it's 2/3 wrong. if you look at as an entirely new choice,it's 1/2. that he threw out a zonk is irrelevent, you know of the 2 doors you have left to choose from, 1 is right and 1 is wrong

tiiznit (5:44:37 PM): it's not an entirely new choice.. the whole basis of the argument is that there is a link

tiiznit (5:44:54 PM): did you try the java thingie btw?

King Nixon (5:45:22 PM): why shoudlnt it be a new choice? he changed the choices so you make a new choice

King Nixon (5:45:34 PM): no i havent yet. i'm being plato =) pure thought, experiment is less fun

King Nixon (5:45:42 PM): (that was plato, wasnt it?)

King Nixon (5:45:52 PM): or socrates? i dont remember

King Nixon (5:45:56 PM): or some other jerk

tiiznit (5:46:03 PM): heh, yeah, i ignored that until i had figured it out myself, and then amused myself by proving it

tiiznit (5:46:07 PM): mm jerk

tiiznit (5:46:38 PM): anyway, you can't look at the whole thing as two choices. if you look at it as being three choices, even when it's narrowed to two, then you can see where the 2/3 / 1/3 comes in

tiiznit (5:47:26 PM): stuff like this was pretty much the entire first three months of my phil class last year. i love that.

King Nixon (5:50:36 PM): but why would you look at it as 3 choices when its narrowed? thats like choosing between 2 closed box and one box you can see is empty. you KNOW the empty one is wrong, so that cant beweighed the same as the other 2

King Nixon (5:52:01 PM): youre choosing between A or (B + C) when you know either B or C is wrong

tiiznit (5:52:06 PM): its weight is that it was C instead of B or B instead of C. in 2/3 of the possible outcomes, the one that was removed is relevant because hte answer is the other. in the third case, it's a random choice, but that's only 1/3

King Nixon (5:52:17 PM): which since youre looking for hte right one, makes them essentially equal one

tiiznit (5:52:46 PM): why do you like arguing the incorrect argument? =)

King Nixon (5:53:08 PM): i dont think the two choices should be related. if he removes 1 of the doors the probability has to change, theres only 2 options left

King Nixon (5:53:48 PM): youve got A B or C. you know C is wrong. its still an option, but you have 50/50 of it being either A or B.

King Nixon (5:54:06 PM): if you chose A or B originally, its still 50/50 between them. otehrwise you'd be affecting it by your choice which is impossible