3 karma · joined November 3, 2020
I know that the prize is not behind door A (because the host showed me the goat there). Given that the prize is not behind door A calculate the probability that it is behind door B. Well that's obviously 1/2. Why? Just use conditional probabilities that's what they are for. (The calculation is trivial: Event X="prize behind B", event Y="prize not behind A", P(X)=1/3, P(Y)=2/3. P(X and Y)=1/3. P(X|Y)=P(X and Y)/P(Y) = 1/2.)
Exactly why this argument is wrong is the subtle part. Of course the argument doesn't put to use part of the information given - which door I chose originally and what contract I had with the host, so this is normally enough to make one uncomfortable, but still we know that, e.g.
"In probability theory, conditional probability is a measure of the probability of an event occurring, given that another event (...) has already occurred." (source: wikipedia)
Having thought about it I find that adding some more precision to the intuitive motivation of conditional probabilities might be helpful. I certainly think twice now before using conditional probabilities to model the real world, but realizing that I should has cost me quite a bit of head scratching.