Originally Posted by
Dubya
Not so, I have to say:
From the outset you have a one in three chance of selecting the door with the car first off.
This means that there is a 67% chance that the car is behind one of the two doors you did not choose and a 33% chance it is behind the one you first chose.
These percentages do no vary after the door with the goat behind it is opened.
We already knew there was a goat behind one of the two doors you did not select; now we just happen to know behind which of the two other doors the goat was hidden.
As the two doors we did not choose initially still have a combined 67% chance of having the car behind them, with the knowledge that the GTI is not behind the door that has been opened, if you swap, your chances of the car being behind your second choice are 67%, not 33% or 50%.
So if you swap, on average, you will win twice as often.