Please Explain (actually explain) The Monty Hall Problem

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Discussion Overview

The discussion revolves around the Monty Hall Problem, a probability puzzle involving a game show scenario where a contestant must choose between three doors, behind one of which is a prize. Participants explore the implications of the initial choice and the information revealed by the host's actions, questioning the relevance of the first choice in determining the final probability of winning.

Discussion Character

  • Debate/contested
  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant argues that the first choice is immaterial and does not affect the outcome of the game, suggesting that the game continues regardless of the initial selection.
  • Another participant counters that the act of the host revealing a losing door provides crucial information that impacts the probabilities involved.
  • A different viewpoint emphasizes that the first choice does not provide relevant information, asserting that the final decision is what ultimately matters.
  • Some participants illustrate the problem using variants with more doors, arguing that the principle remains the same and that switching doors increases the chances of winning.
  • There is a contention regarding the assumption of equal probability in the final choice, with some asserting that the probabilities are not equiprobable after the host's reveal.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of the first choice in the Monty Hall Problem. While some believe it has no bearing on the outcome, others argue that it does provide information that influences the final probabilities. The discussion remains unresolved, with multiple competing perspectives present.

Contextual Notes

Participants reference various scenarios and analogies to illustrate their points, indicating that the discussion is nuanced and depends on interpretations of probability and information theory. There are unresolved assumptions regarding the nature of the game and the implications of the host's actions.

  • #61
sysprog1 said:
None of Monty's motions changes the 1/3 chance you have of having picked the prize door.

sysprog1 said:
If you lose when you switch, it's because you picked the prize door and switched; but whenever (2/3 chance) you don't pick the prize door, switching always wins.
That's a good way to look at it. Your original door still has 1/3 chance of winning. That leaves 1-1/3 = 2/3 chance and only one door for the 2/3 chance. So it is best to switch to that door.

ADDED: What makes your door different from the other remaining unopened door is that the rules didn't allow Monte to open your door. So we learn nothing from the fact that he didn't open your door. On the other hand, he was allowed to open the other remaining unopened door if it doesn't have the prize. So we should adjust our judgement about that door.
 
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  • #62
In such a case one may resort to experiment.
 
  • #63
sysprog1 said:
Monty doesn't open the door with the car, and he doesn't open the contestant's selected door. So he always opens a non-car non-selected door.
This is the critical assumption/characteristic of the game. First, Monty's pick is not random (he never reveals the car); and second, he always offers a switch (not only when he knows you have the car!).

A more general problem would involve Monty opening a door and offering a switch with probability ##p## if the contestant has picked the car door, and a probablity ##q## if the contestant has not picked the car door.

In that case, given that a switch has been offered, sticking wins with probablity ##\frac{p}{p + 2q}##; and switching wins with probablity ##\frac{2q}{p + 2q}##.

And it's better to switch if ##2q > p##.

The problem you are analysing has ##p = q = 1##.

If ##p = 1, q = 0##, then it's better to stick.

And, if ##p = 1, q = 1/2##, then sticking and switching have equal likelihood of winning. I like this twist!
 
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