> Every time I hear this one I scratch my head in confusion.
>
> At the beginning, you have a 1 in 3 chance of guessing the right door.
>
> You choose,  Monty always opens one door.  There are two
> doors remaining
> (including the one you choose).  You now have a 1 in 2 chance
> of having the
> right door.
>
> If you choose the other, you still have a 1 in 2 chance of
> having the right
> door.

Not quite. It's easiest to get to the correct answer if you turn the
probability around: when you have picked your door out of the three
available, the odds are 2:1 that your door is *not* the one hiding the Big
Deal. Monty opens one of the other doors. That does nothing to change your
door's chances; it is still 2:1 that your door does not hide the Big Deal.
Since there's only one unopened door left, the odds are, by definition, 2:1
that that door *does* hide the Big Deal. Make the switch.


Steve Dirickson   WestWin Consulting
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