Even though every school has a formula for how many will accept its offer of admission, it's impossible to truly know how many people will decide to go, especially in the current economic climate. If they admit 400 people with a target enrollment of 200 (hypothetical), it still might be possible that there are 250 people that are unable to go for financial or other reasons. Hence, the wait list. I don't know that there is really a trend as to how many people get into a school off of the wait list; I'd imagine it varies year to year.