Originally Posted by Echodork

Well, I can tell you that this isn't true. To have a 100% chance of duplicate birthdays, you'd need 366 people (not counting leap year birthdays). The math is a bit tricky and I don't feel like working it out right now, but I'm willing to accept that 30 people have a 90% chance for duplication

I think the forumula for finding the probability of a duplicate birthday is:

1 / (1/365 + 1/364 + 1/363...1/n-1) where n is the number of people in the room. I'm not sure, and that tends to disprove the theory that 30 people have a 90% chance, since using 30 for n results in a chance of about 9%, not 90%. But I got my degree in psych, any math students out there want to comment?