
In a comment on my post Top Secret Tips For Winning Game Shows, Marelisa the abundance queen reminded me of the Monty Hall paradox, which deserves a post of its own.
This problem became famous in 1990 when Marilyn vos Savant wrote about it in Parade magazine. This is how she stated it:
“Suppose you’re on a game show, and you’re given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what’s behind the doors, opens another door, say No. 3, which has a goat. He then says to you, ‘Do you want to pick door No. 2?’ Is it to your advantage to switch your choice?”
She said that you should switch, because switching will give you a 2/3 chance of winning the car, compared to 1/3 if you stick with door #1.
Not exactly intuitive, is it?
She received more than 10,000 letters from people (including 1,000 Ph.D.s) saying that she was wrong (and presumably saying the odds were 50/50 either way). She also got a letter from me, saying something different (we’ll come back to this).
Russian roulette
Here’s another way of looking at it. Let’s say you’re playing Russian roulette with one other person, using a gun with 6 chambers and 1 bullet. You spin the cylinder and you’re about to pull the trigger to fire chamber #1, which has a 1/6 chance of having a bullet.
But before you do, the other guy asks for the gun, saying he’ll fire four of the other chambers first. So he takes the gun and pulls the trigger 4 times, firing cylinders #2, #3, #4, and #5. Luckily for him, they were all blanks. He gives the gun back to you, and asks if you want to stick with your original cylinder #1, or switch to cylinder #6.
Because he didn’t know where the bullet was, it doesn’t matter if you switch. It’s a 50/50 chance either way.
But let’s back up and try something else. You’re about to fire chamber #1 when the other guy asks for the gun, saying he’ll fire four of the other chambers first. He takes the gun, but this time, he swings out the cylinder to see where the bullet is. Then he fires chambers #2 and #3, skips chamber #4, and fires chambers #5 and #6, all of which are blanks. He then asks if you want to stick with cylinder #1, or switch to cylinder #4.
Does it matter if you switch now? You bet! He skipped chamber #4 for a reason. And if the bullet was in any of the other chambers, you can bet that he would have skipped those instead. There’s a 1/6 chance that the bullet was in chamber #1 and he just skipped a random chamber to mess with you. But there’s a 5/6 chance that the bullet was not in chamber #1, and he specifically fired all the remaining blank chambers, leaving just the bullet.
Just like you should stick with chamber #1 to avoid the bullet, you should switch to another door to win the car.
The real answer
However, one of the reasons there was so much debate about this problem is that it’s ambiguous, and that’s what I said when I wrote to her. We just don’t know enough about the host’s reasoning for picking the door to open. For example:
- If the host had decided that he was going to open door #3 no matter what, and it just happened to be a goat, your odds are 50/50 with either door.
- If the host wanted to be nice and only offer you the chance to switch if your original choice was wrong, and only open a door containing a goat, you’d have a 100% chance of winning by switching.
- If the host wanted to be a jerk and only offer you the chance to switch if your original choice was right, you’d have a 100% chance of losing by switching.
To spell out Marilyn’s assumptions, the problem would read like this:
“Suppose you’re on a game show, and you’re given the choice of three doors: Behind one door is a car; behind the others, goats. The host, who knows what’s behind the doors, has decided in advance that after you pick a door, he’ll open another door and give you the choice of switching to the remaining door. He’ll decide which door to open using this logic: if you pick the door with the car, he’ll open another door at random; if you pick a door with a goat, he’ll open the other door with a goat.
You pick door No. 1, and the host opens door No. 3, revealing a goat. He then says to you, ‘Do you want to pick door No. 2?’ Furthermore, he explains his logic in choosing to open door No. 3, and you know he’s telling the truth. Is it to your advantage to switch your choice?”
It kind of ruins the fun to spell out everything like that, but you can’t talk about who’s right and who’s wrong if the problem is ambiguous! When the problem is stated this way, the answer is that you should switch, because door #2 has a 2/3 chance of having the car. The way Marilyn stated it, it’s too ambiguous to answer unless you make some assumptions.
BTW, did you assume that the car is better than a goat? In a 1999 auction, someone paid $80,000 for a full-blood adult South African Boer goat. So maybe you shouldn’t switch after all!