In fact, it is immensely important that the problem states that Monty Hall always reveals a losing door. If the problem merely states a particular series of ev

In fact, it is immensely important that the problem states that Monty Hall always reveals a losing door. If the problem merely states a particular series of events without specifying the rules -- as both Jeff's problem statement and Marilyn's problem statement do -- you can't know the probabilities. Suppose, for example, that Monty has the policy of only revealing a losing door if the player has chosen the winning door; if, on the other hand, the player initially chooses a losing door, then Monty reveals no door and makes no offer to switch, sticking the player with the loss. In that case, if you encounter the same sequence of events in Jeff's problem description, then switching always has zero chance of winning. With the opposite assumption -- that Monty only reveals a losing door if the player has initially chosen a losing door -- switching always has a 100% chance of winning. Both Jeff and Marilyn nevertheless press forward with the unstated assumption that Monty has no choice in the matter. It turns out that this assumption is not only faulty, but it's also unrealistic: on the actual game show, Monty was under no such obligation. I'm a little surprised that Jeff, who apparently has read up a little on the topic, didn't bother to grab the much better problem specification from the Wikipedia article.