If I recall correctly it's by MCAT.
What I do know is that first they admit the high waitlisters, then the medium waitlisters, and lastly the low waitlisters. Traditionally, I think no one from the low waitlist makes it in for the current year. I remember Dr. Haight indicating that last year nobody from the medium waitlist group got in and not everyone from the high waitlist group got in.
Good luck!