The Monty Hall Problem: A Bayesian Approach

How I Attacked a Probabilistic Puzzle

Posted by Chi Zhang on May 6, 2016

1. How I Came Across Monty Hall

It is actually pretty straightforward: a similar problem showed up in Problem Set 1 of the course Introduction to Data Mining. It all came as follows:

There are three boxes. Only one box contains a special prize that will grant you 1 bonus points. After you have chosen a box B1 (B1 is kept closed), one of the two remaining boxes will be opened (called B2) such that it must not contain the prize (note that there is at least one such box). Now you are are given a second chance to choose boxes. You can either stick to B1 or choose the only left box B3. What is your best choice?

Well, for those of you familiar with Monty Hall Problem, this actually IS one with only modifications of the background.

FYI, the Monty Hall Problem [1] is first proposed in the Parade magazine.

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. 2, which has a goat. He then says to you, “Do you want to pick door No. 3?” Is it to your advantage to switch your choice?

So, anyway, what is your choice?

2. Bayesian Statistics

Alright, before we proceed to the solution, let’s first have a brief review of Bayesian Statistics.

The following equation, or Bayes’ Theorem, lies at the heart of Bayesian Statistics,

It basically is just a reformulation of the condition probability equation,

Under independence, the equation just reduces to

or equivalently,

As for $P(B)$,

OK, now that you are equipped with fundamentals of statistics, let’s move on!

3. The Bayesian Approach

Let’s view the Monty Hall problem in its original scenario.

To decide whether we should switch, we should compare probabilities of $P(No. 1 = car \mid No. 2 = goat)$ and $P(No. 3 = car \mid No. 2 = goat)$. And these are just conditional probabilities that require the Bayes’ Theorem. Note that

Thus, we only need to work out $P(No. 1 = car \mid No. 2 = goat)$.

Using the Bayes’s Theorem,

The probability of No. 1 containing the car is

while the probability of the door the host opens containing the goat given that your choice contains the car could be easily derived as

Well, until now, nothing fancy. But here comes the trickiest part.

What is $P(No. 2 = goat)$?

One who thinks little of it might carelessly concludes $2 \over 3$ since the probability of every door containing the car is $1 \over 3$ and that of a goat is just the complement.

Excuse me, but that’s not true.

Notice in the statement that the host always opens a door that does not contain a car.

Therefore the probability is actually $1$.

As a result,

AH! It seems that we should switch!

Quite counter-intuitive!

4. The Frequentist Approach

It was actually a bit confusing when I made my first attempt to understand it from a Bayesian view, although it turned out easier if you simply listed all cases and counted the number of wins.

Let me show you.

Behind No. 1 Behind No. 2 Behind No. 3 Result if stick Result if switch
Car Goat Goat Win Lose
Goat Car Goat Lose Win
Goat Goat Car Lose Win

Table from [1]

As illustrated above, $P(No. 1 = car \mid Door = goat) = {1 \over 3}$.

5. Naive Generalization

Now that we have worked out the case when there are only 3 doors with only 1 containing the prize, it’s natural that we seek the solution when there are more doors and more than 1 prizes.

Similarly, if there is 1 prize and n doors, we should again switch since the Bayes’ Theorem tells that the probability of wins remains $1 \over n$.

But the problem becomes a little bit complicated when there are more than 1 prizes.

Suppose there are m prizes and n doors. Then the probability of winning if you stick turns to be

If this is larger than $1 \over 2$ then we stick, otherwise we switch.

References

[1] Wikipedia. Monty Hall problem.