This is one for math geeks with a Facebook account. I recently heard of an interesting probability problem called the Birthday Paradox. I think it was on the excellent Five Numbers series on BBC Radio 4 (or one of the 2 follow-up series). Anyway, the problem states that if you put 23 (or more) randomly selected people in a room, there is more than a 50% chance that at least 2 people will share the same birthday. The solution is not a paradox in the logical sense, it just contradicts our intuition.
Now for the Facebook link. If you have an account you probably know about the birthdays page where you can see a list of all your friends’ birthdays. I checked it the other day and was surprised to find that 3 pairs of my friends shared the same birthday. That is, before I remembered the birthday paradox and that with my 40 Facebook friends there is a 90% chance that at least 2 of them share the same birthday.
If, unlike me, you are uber popular on Facebook and have more than 100 friends, the probability should be more than 99.99996%!
(OK, I did say that this one was for math geeks…)