Originally Posted by woj

1. you pick a door, 1/3rd of the time it's a winner... 2/3rds it's not...
2a. IF the chosen door is a winner (1/3rd chance) and you switch = you lose
2b. IF the chosen door is not a winner (2/3rd chance) and you switch = you win

so by using the "switching" strategy, you win 2/3rds of the time...