[sepp2k]

raziel_, on 22 September 2012 - 05:13 PM, said:

Maybe im just stupid over here but if one of the door is opened then dont you have 2 choices now?

Yes, you have the choice to stay with your original choice or to switch.

Quote

and your chance is 50/50?

No. Think of it this way: If you picked one of the two wrong doors at the beginning, you'll always get to the correct door if you switch (because the second wrong door will have been eliminated). If you picked the right door at the beginning, you'll get a wrong door if you switch. So what is the chance that you win if you switch? 2/3 because that's the chance of picking a wrong door at the beginning.

[baavgai]

Your answer is intuition, and what most people believe, but not math. Indeed, sometimes it's so hard to convince someone of this that you have to write a computer based simulation.

Think of it this way, your first guess has a one in three chance, right? With two in three being the chance of not guessing right. When you switch your answer, you essentially have chosen two out of three, not one out of three.

[jon.kiparsky]

raziel_, on 22 September 2012 - 10:13 AM, said:

Maybe im just stupid over here but if one of the door is opened then dont you have 2 choices now? and your chance is 50/50? and hows that if one of the doors is out of the game is still counted as a choice?

You're wrong, but that doesn't make you stupid.

Yes, you have two choices, but the odds are not 50/50 that the prize is behind one of your two choices. Remember when you picked one door from three? When you picked that door, your odds were 1/3, and it was 2/3 that the prize was behind one of the other two doors.

Imagine we play the game a little differently. This time, the host does not open a door, but asks you if you want to take both of the other doors, or the one you started with. What are your odds this time, if you switch? If you stay?

[Skydiver]

I'm beginning to grok the result, but just to make sure, I understand things, I'll ask a slightly different question:

If presented a red pill, a blue pill, and a green pill where one of the pills is poisonous. After I pick a pill, and one other pill is shown to be a placebo, then I should not change my mind. Is this correct?

[jon.kiparsky]

That is correct. The odds of you picking the poisoned pill the first time are 1/3. The odds of your not having picked it are 2/3. That means if you switch, you go from 1/3 chance of getting the poison to 2/3 chance.

Of course, someone who presents you with that sort of puzzle is likely to know that you're familiar with this problem and therefore you're into a mindfuck situation. At that point, your best bet is to overpower the son of a bitch and cram all three pills into him, and hold his mouth and nose until he swallows. Your odds are much better in that scenario, assuming you get the advantage of surprise.

[Skydiver]

Where is iocane powder when you need it? (Unfortunately, only a choice between two rather than one out of three.)

This post has been edited by Skydiver: 22 September 2012 - 11:24 AM

[Momerath]

And this problem is why I can no longer watch Deal Or No Deal. They consistently misstate the odds of having the $1,000,000 case, and I yell at the TV. My wife doesn't like it when I yell at the TV

And if you are ever on the show and you are down to the last two cases and they ask if you want to switch your case, you say YES!

[foohoo]

The logic works on the fact that your beginning choice has a 2 in 3 chance of being wrong (i.e. the goat). When you get to the second stage and the gameshow host removes a door (which is always a goat i.e he can't remove the good prize) it means, as you have most likely selected a goat in the first stage, the remaining door holds the good prize and you should swap.

If it got to the second stage and the host removed one of the remaining doors at random (so he could actually remove the good prize) then you are back to a 50/50 choice.

- at least this is how I understand it

[jon.kiparsky]

foohoo, on 28 September 2012 - 08:28 AM, said:

If it got to the second stage and the host removed one of the remaining doors at random (so he could actually remove the good prize) then you are back to a 50/50 choice.

- at least this is how I understand it

I'm with you up to this part. In this case, you have the same odds of getting the prize whether you switch or stay, but it's not 1/2. There's a 1/3 chance that the prize has been removed, so you have 1/3 chance of getting it if you switch or if you stay.