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The famous Monty Hall problem in the field of statistics goes like this: Monty Hall is a game show host. You are given a choice of three doors. One has a car behind it, the other two have goats. If you pick the door with the car, you win it. Your odds are 1-in-3.

So you pick a door, but before it opens, Monty opens one of the other two doors to reveal a goat. He asks if you want to switch from the door you initially picked to the other closed door. Your brain says the odds are the same for any closed door, so you stay. But in fact, the odds are twice as good if you switch doors.

You can see the math of it here. But if you are normal, you'll never reconcile in your mind how one closed door could have better odds than the other. If there are two closed doors remaining, how can the odds be anything but 50-50?

This reminds me of the Schrodinger's Cat thought experiment in which a cat in a sealed box (presumably with air holes) exists in a state of being simultaneously alive and dead depending on the results of a randomized event happening inside the box. How can a cat be alive and dead at the same time? Math says it can happen, my brain says no.

The pattern recognition part of my brain is connecting the Monty Hall problem with the Shrodinger's Cat thought experiment because both situations feel like proof that our brains are not equipped to understand reality at its most basic level.

Most of us accept the idea that math is a better indicator of truth than our buggy personal perceptions. Math doesn't lie, but our brains are huge scam artists. The Monty Hall problem and Schrodinger's Cat are examples in which our perceptions of reality and the math of reality disagree in a big way. It makes me wonder how much of the rest of my so-called reality disagrees with math without me knowing.

If I were programming a computer simulation full of artificial humans who believe they are real, I would need to take some shortcuts in creating their context and history. It would be nearly impossible to invent consistent histories for seven billion people spanning back to the primordial ooze. A far smarter approach would be to craft the history as you go, based on the present, in whatever minimum way is necessary to make all histories consistent.

For example, let's say you learn that you are the grand winner of a lottery. At the moment you realize you are the big winner, history becomes limited to only the possibilities that got you to that winning moment. Before you learned you were a winner, the reality at the lottery headquarters was only a smear of possibilities - like Shrodinger's Cat - where you were both a winner and a loser, just like everyone else. As soon as you learn you won, your history and everyone else's harden to conform to it. No one else can perceive that they won the grand prize in that particular lottery.

If I were the programmer of this simulation that you call your reality, I would make the history dependent on the present just to streamline my work. All I need from my fake history is that it is consistent with all the other fake histories so there is no "tell" left by the programmer.

I realize the simpler explanation for my confusion about Monty Hall and Schrodinger's cat is "Math be hard." But I like the psychological freedom of feeling as though I am the author of my own history and not its bitch.

Here's the cool part: I get to keep my interpretation of reality - in which my history is a manufactured illusion - until something in my present experience is inconsistent with that view.

Recently I heard of two senior citizens with mild dementia who became friendly at a senior care facility. Their fragile minds concocted an elaborate history of being childhood acquaintances that had found each other through fate. No one tries to dissuade them of this illusion because it works for them. They successfully rewrote their histories without any repercussions.

I wonder how often the rest of us rewrite our histories. Our only limitations are that our new histories have to be consistent with whatever scraps of history have already hardened.

 
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Mar 30, 2013
@AtlantaDude
If you want to understand why, all you have to do is read my last post. :-)
 
 
Mar 30, 2013
OK. I have finally come around on this. I ran my own simulation, and switching was better. Still couldn't understand why. Then I looked at the inverse - what if I was a dedicated switcher and I wanted to lose. When Monty reveals a goat or mule, he takes away one of my chances to lose, so he made it harder for me to lose, and thus easier for me to win.

And, despite all that, I still don't really understand "why" this is the answer and my previous analysis is wrong. The only thing I can say with certainty is that I remember why I always hated probability classes.
 
 
Mar 29, 2013
To simplify further, let's just look at the switcher scenarios
1. Pick-Car, Reveal-Goat: You Switch to Mule - Lose
2. Pick-Car, Reveal-Mule: You Switch to Goat - Lose
3. Pick-Goat, Reveal-Mule: You Switch to Car - Win
4. Pick-Mule, Reveal-Goat: You Switch to Car - Win
5. Pick-Goat, Reveal-Car: Game Over - Lose
6. Pick Goat, Reveal Goat: Game Over - Lose
7. Pick-Mule, Reveal-Car: Game Over - Lose
8. Pick Mule, Reveal Mule: Game Over - Lose
9. Pick Car, Reveal Car: Game Over - Win

Switcher has 3 wins in 9 scenarios. Or, if you want to just concentrate on the situations in which Monty opens a non-chosen losing door - that is scenarios 1 thru 4 and Switcher has 2 wins in those scenarios.

Now let's look at the possibilities just for a person who keeps his original choice
1. Pick-Car, Reveal-Goat: You Keep Car - Win
2. Pick-Car, Reveal-Mule: You Keep Car- Win
3. Pick-Goat, Reveal-Mule: You Keep Goat - Lose
4. Pick-Mule, Reveal-Goat: You Keep Mule - Lose
5. Pick-Goat, Reveal-Car: Game Over - Lose
6. Pick Goat, Reveal Goat: Game Over - Lose
7. Pick-Mule, Reveal-Car: Game Over - Lose
8. Pick Mule, Reveal Mule: Game Over - Lose
9. Pick Car, Reveal Car: Game Over - Win

Keeper also has 3 wins in 9 scenarios. Or, if you want to just concentrate on the situations in which Monty opens a non-chosen losing door - that is scenarios 1 thru 4 and Keeper has 2 wins in those scenarios.

So the odds are the same for Keeper and Switcher, regardless of whether you consider just the cases where Monty opens a loser door, or if you look at all 9 scenarios (3 choices by you * 3 choices by Monty)

[The fascinating thing is that opposing arguments are both 100% airtight. It's only the human impulse to pick sides that makes us think there is a clear winner as far as the logic goes. But I believe the simulations on this always support the better odds of switching. -- Scott]
 
 
Mar 29, 2013
I will boil it down for the doubters. Rather than just giving me a thumbs down, please tell me what other possible scenarios exist, other than the 9 below.

1. Pick-Car, Reveal-Goat: You can either Keep Car or Switch to Mule
2. Pick-Car, Reveal-Mule: You can either Keep Car or Switch to Goat
3. Pick Car, Reveal Car: Game Over - Win
4. Pick-Goat, Reveal-Mule: You can either Keep Goat, or Switch to Car
5. Pick-Goat, Reveal-Car: Game Over - Lose
6. Pick Goat, Reveal Goat: Game Over - Lose
7. Pick-Mule, Reveal-Goat: You can either Keep Mule, or Switch to Car
8. Pick-Mule, Reveal-Car: Game Over - Lose
9. Pick Mule, Reveal Mule: Game Over - Lose

There are no other branches to this tree. There 9 scenarios. You have three chances to win if you start the game with a Keep strategy and three chances to win with a Switch strategy
 
 
Mar 29, 2013
@Melvin1 - That doesn't matter. Instead, the fact that Monty intentionally opens a goat/mule door is the reason why there is no new information revealed when he does the reveal. However, if he randomly opened another door and got a goat, then there would be Bayesian updating and the two problems - choosing before the reveal, choosing after the reveal - would not be independent.

As it is, the two problems are completely independent. The first problem is like you are going to walk into one of two casinos. The first casino has a car and goat and the second casino has a car and a mule. Finding out which casino you walked into has no bearing on your odds of winning.
 
 
+2 Rank Up Rank Down
Mar 29, 2013
@AtlantaDude: Scott didn't explicitly say it, but the puzzle presumes that Monty intentionally opens a goat door.
 
 
Mar 29, 2013
Here is the logic explaining why switching does NOT improve your chances:

The first thing to do is avoid confusion caused by there being two goats. There are two bad outcomes, and for clarity, it is better if we make one a goat and the other a mule.

Now, If we start with three equal possibilities on your first pick it can either be a goat, a mule or a car, and we assume that you have an equal chance of picking any of them. We also know that Monty will ALWAYS reveal a bad prize, regardless of what your initial pick was. Therefore, the only scenarios that exist are as follows.

Pick-Car, Reveal-Goat: Keep Car, Switch Mule
Pick-Car, Reveal-Mule: Keep Car, Switch Goat
Pick-Goat, Reveal-Mule: Keep Goat, Switch Car
Pick-Mule, Reveal-Goat: Keep Mule, Switch Car

As you can see there are two “Keep” outcomes that result in a car, and there are two “Switch” outcomes that result in a car. So keeping and switching both have an equal opportunity to win, and the probability of winning is 2 out of 4, or 50%. This is the probability problem framed from the point AFTER he opens the door.

Now, what if you tell me that Monty does not ALWAYS reveal a bad prize, but that he could also reveal an unchosen door with a car in it? If that is the case, you have to add the following 2 scenarios where he reveals the car after your initial choice

Pick-Goat, Reveal-Car: Game Over-Lose
Pick-Mule, Reveal-Car: Game Over-Lose

Adding those scenarios, gives you the following possibilities

Pick-Car, Reveal-Goat: Keep Car, Switch Mule
Pick-Car, Reveal-Mule: Keep Car, Switch Goat
Pick-Goat, Reveal-Mule: Keep Goat, Switch Car
Pick-Mule, Reveal-Goat: Keep Mule, Switch Car
Pick-Goat, Reveal-Car: Game Over - Lose
Pick-Mule, Reveal-Car: Game Over - Lose

There are still two winning Keep scenarios and two winning Switch scenarios, so keeping and switching have an equal chance of winning.

Finally, let’s throw in the option of Monty not doing a reveal. Instead he simply opens the door you choose initially. Those three scenarios look like this

Pick Car, Reveal Car: Game Over – Win
Pick Goat, Reveal Goat: Game Over – Lose
Pick Mule, Reveal Mule: Game Over – Lose

Now, we can combine all these scenarios to look at every outcome. This list comprises all potential outcomes before you make your initial choice.

1. Pick-Car, Reveal-Goat: Keep Car, Switch Mule
2. Pick-Car, Reveal-Mule: Keep Car, Switch Goat
3. Pick-Goat, Reveal-Mule: Keep Goat, Switch Car
4. Pick-Mule, Reveal-Goat: Keep Mule, Switch Car
5. Pick-Goat, Reveal-Car: Game Over - Lose
6. Pick-Mule, Reveal-Car: Game Over - Lose
7. Pick Car, Reveal Car: Game Over – Win
8. Pick Goat, Reveal Goat: Game Over – Lose
9. Pick Mule, Reveal Mule: Game Over – Lose

If you go into this game with a “Keep” Strategy you have 3 chances to win (1, 2 and 7 above) out of 9 total for a 33% probability. If you go in with a “Switch” strategy, you also have three chances to win (3, 4 & 9). So there is not advantage in switching. This is the probability problem BEFORE he opens door, and you have a 33% chance of winning, regadless of whether you go in with a Keep or a Switch strategy.

So if you view the problem from the beginning, you have 33% chance with a Keep or Switch strategy. If you view the problem from the point after a bad prize is revealed, you have a 50% chance with a Keep or Switch strategy.

The confusion comes from two areas - one is mixing up or not counting all the goat outcomes, and the other is mixing up the probability as viewed from the beginning with the probability as viewed from after a door is opened.
 
 
+2 Rank Up Rank Down
Mar 29, 2013
It's been said a few times, but I'll jump on the bandwagon:
The Monty Hall problem wasn't that hard for me to figure out -- after my initial 50:50 "obvious" answer. Assuming, of course, that Monty intentionally opens a goat door, he's giving additional information about the two doors you didn't choose, doubling those odds. No magic or reality-bending.

Schroedinger's Cat is a reductio ad absurdum showing the limits of a model (the Copenhagen interpretation) of quantum physics. His argument was that a cat is clearly NOT both alive and dead, and therein lies the weakness of applying quantum physics to the macro world.

That said, quantum physics is indeed difficult for most of us to understand - almost magical. But so were heliocentrism and gravity at one point. We've even become pretty comfortable with electricity (the most useful way of doing so for me was with plumbing analogies).

You raise an interesting question about how we rewrite histories, but you've muddied it up with incorrect examples. Those are just logic puzzles, not alternate realities.
 
 
Mar 29, 2013
I am afraid that Scott as intentionally or unintentionally bamboozled his readers with this one. The Monty Hall problem is NOT a two-thirds chance of winning if you switch.

The initial problem, pick one out of three, has actually no bearing on the second problem which is pick one out of the two remaining. As soon as he opens a goat door, the original problem is over. Then you are left with a new problem, which is simply that is simply keep door / switch door, each having an equal chance of having a car.

The problem with the explanation, is that it assumes there is some dependency between the two problems, and there would be if Monty only revealed a door when you picked the car. However, he will always remove a goat door, regardless of what you originally pick. Therefore, that goat door does not "lend" its probability to the other non-chosen door. It simply reduces the denominator for both remaining doors' probability from 3 to 2.

For a better explanation see: http://ablestmage.wordpress.com/2007/11/30/the-classic-monty-hall-problem-gets-goatsed/
 
 
Mar 29, 2013
I expect that thousands of years ago most humans would have summarily rejected the idea that the Earth is a sphere that orbits the Sun because it was so completely counter-intuitive. Our intuition has evolved.
 
 
Mar 29, 2013
[Pretending, or deluding yourself into believing, that reality does not exist independent of your perception of it is no more than a delusion of grandeur. Reality may suck, but reality it still is.]

[And you know that how? -- Scott]

Reality is what you can get away with. If you can't get away with it it isn't real and has to be dealt with. Saying it isn't real doesn't change that, so the proper course of action is to accept what you perceive as real and deal with it accordingly.
 
 
0 Rank Up Rank Down
Mar 29, 2013
My 52 yr old sister changes our mutual childhood memories regularly to benefit and support her delusional behavior to rationalize her current abusive behavior. Or maybe it is me that has a different memory of mutually experienced events. Or maybe we're both wrong...point being that the human brain is inherently imperfect and we use memory to adapt to current situations and when it is not a life or death adaptation, our brain plays games with us. I'll take door #2.
 
 
-4 Rank Up Rank Down
Mar 29, 2013
Concerning Schroedinger's cat, leaving aside what his intentions were or not... Who put the cat in the box in the first place? Surely he knows if it was alive or dead or not - it didn't just appear there!

You could give it a little shake. If it's alive, you'll know without having to open it. Believe me, you'll know.

Seems kind of odd to think that the only way you could know if something is alive or dead is by looking at it with human eyeballs when other sense we have could easily answer it for us. Have you SMELLED a dead cat?

The cat had a fixed state long before it entered that box; just because YOU don't know what state it was in doesn't mean SOMEONE doesn't. Like the cat. Or the janitor who didn't want to fill out "dead animal" paperwork.

What if you had a box with an alive or dead cat behind one of three doors and a gameshow host who was clearly a dog person... Would it be on TLC? Or is that more of an A&E channel kind-of show?

This was either very wise, or mindbogglingly ignorant... I think I will choose wise.

 
 
+2 Rank Up Rank Down
Mar 29, 2013
Scott, You made my head hurt then I realized you're my long lost brother. Remember how you were always extrapolating theories from the minutiae of life at the dinner table and mom would always tell you to be quiet and drink your milk? Whew! Glad I remembered, it all makes sense now.
 
 
Mar 29, 2013
I've had two college-level professors tell me that the whole Schrodinger's cat thing is mainly an example of how great scientists sometimes majorly misunderstand each other. You may ahve made a valid point, but I wouldn't use Schrodinger's cat as evidence of it.
 
 
Mar 29, 2013
[I wonder how often the rest of us rewrite our histories.]

Constantly. Usually along the lines of "I can assure you officer that I was wearing trousers when I entered this park. Right up until the point I was abducted by aliens. When I awoke I was as you see me now. Have you read any Schrödinger lately?"
 
 
+1 Rank Up Rank Down
Mar 29, 2013
It's not hard.

When you pick a door you have a 1 in 3 chance of getting it right, 2 in 3 chance you got it wrong.

When the host opens a door and you switch, you move over to the 2 in 3 section.

 
 
Mar 29, 2013
There's nothing wrong with the Monty Hall problem. This sort of chance estimation is not a fundamental part of reality (except perhaps in quantum scenarios); it is a tool for coping with ignorance. The chance estimates change because our data changes. However the hard reality is that it either is or is not behind a particular door. It might not feel sensible to you, but there's no reason to except our brains are flawless. It is evidence of jack squat.
 
 
Mar 29, 2013
Scott,
Congratulations on your comment on Telanis' post. Could not have put it better.

But now after grgeil's post I understand it.

There are two options open to you. a)do not switch or b)switch

A.
If you do not switch, there is a total of 1/3 probability that you will get the car. Monty subsequently opening a door does not change the odds, since you have already chosen one out of three doors.

B.
If you do switch, you will surely win if you started with a goat door. And the odds of starting with a goat door are 2/3.

Therefore B. has double the probability of A.

Thanks grgeil.
 
 
+5 Rank Up Rank Down
Mar 29, 2013
People overthink the Monty Hall Problem with too much math, probability and percentages. It is quite simple...if you picked the car initially (33.33% chance) you will lose the car. If you picked a goat initially (66.67% chance) you are going to get the car. For the percentage lovers: 100% of the time that you initially picked the car you will lose it. 100% of the time that you initially picked the goat you will win the car instead.
 
 
 
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