Monty Hall problem

The Monty Hall problem, also known as the three doors problem or the quizmaster problem, became famous in 1990 on the publication of an article in Parade magazine in the popular weekly column "Ask Marilyn". The column's author Marilyn vos Savant was reputedly, at the time, the person with the highest IQ in the world. The problem is named after the stage-name of an actual quizmaster, Monty Halperin, on a long-running 60's TV show, though it seems that the events related in the Monty Hall Problem never actually took place in reality.

Rewriting in her own words a problem posed to her by a correspondent, a Mr. Craig Whitaker, Marilyn asked the following:

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?

Almost everyone, on first hearing the problem, has the immediate and intuitive reaction that the two doors left closed, Door 1 and Door 2, must be equally likely to hide the car, and that therefore there is no point in switching. However, some careful thought shows that this immediate reaction must be wrong. Consider many, many repetitions of the game, and suppose (for the sake of argument) that the car is hidden each time anew completely at random behind one of the three doors, while the contestant always starts by picking Door No. 1 (perhaps, because "1" is his favourite number). In the long run, the car will be behind his initially chosen door, Door 1, on one third of the repetitions of the game. If he never switches doors, he'll take the car home with him on precisely those repetitions of the show where his initial choice was actually right - that's one third of the time. On the other hand, if he always switches doors when offered the choice (and that will sometimes be to Door 2 and sometimes to Door 3, depending on which door is opened by the host each time), he'll go home with the car two thirds of the time - he'll win the car by "switching" exactly on every occasion when he would not win it by "staying".

Note that we are assuming, as most readers do and as vos Savant later explained was her intention, that whatever choice is initially made by the contestant, the host is surely going to open a different door revealing a goat and offer the option to switch. One can say that in effect, when the contestant initially chooses Door 1, the host is offering the contestant a choice between his initial choice Door 1, or Doors 2 and 3 together. It certainly seems that a wise contestant will accept the offer to switch.

Here is a more detailed analysis. In this solution we use "probability" in the Bayesian or subjectivist sense; that is to say, all probability statements are supposed to reflect the state of knowledge of one person, and to be specific, of a contestant on the show who initially knows no more than the following: he'll choose a door; the quizmaster will thereupon open a different door revealing a goat, and make the offer that the contestant may switch to the third door. For this contestant, initially all doors are equally likely to hide the car. Moreover, if he chooses any particular door, and the car happens to be behind that particular door, then for this contestant the host is equally likely to open either of the other two doors.

Our tabula rasa contestant initially chooses door number 1. Initially, his odds that the car is behind this door are 2 to 1 against (it is two times more likely for him that his choice is wrong than that it is right).

The host opens one of the other two doors, revealing a goat. Let's suppose that for the moment, the contestant doesn't take any notice of which door was opened. Since the host is certain to open a door revealing a goat whether or not the car is behind Door 1, this new information - that a door is opened revealing a goat - does not change the contestant's odds that the car is indeed behind Door 1; they are still 2 to 1 against. The contestant should switch.

Many authors essentially stop at this point. Some however effectively add one further step in the argument.

Our imaginary contestant also gets informed that the specific door opened by the host is - let's say - Door 3. Does this piece of information influence the contestant's odds that the car is behind Door 1? No! From our contestant's point of view, the chance that the car is behind Door 1 can't depend on whether the host opens Door 2 or Door 3 - the door numbers are arbitrary, exchangeable, mere labels. (Technical aside: a professional mathematician will here refer to the mathematical concept of symmetry and use the law of total probability to show how symmetry leads to statistical independence between the events "Car is behind Door 1" and "Host opens Door 3", when it is given that the contestant chose Door 1).

Therefore, also knowing that the host opened specifically Door 3 to reveal a goat, the contestant's odds on the car being behind his initially chosen Door 1 still remain 2 to 1 against. He had better switch to Door 2.