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Birthdays Aren't Meant to be Shared
Yesterday we asked how many people you need to have at least 50% chance of two people sharing a birthday. The “Birthday Problem” is actually well known in probability theory: According to the pigeonhole principle, the probability reaches 100% at 367 days since there are 366 days (including February 29). If each day is equally likely to be a birthday, you reach 99.9% probability with only 70 people, and 50% probability at 23 people. (It’s a whole lengthy formula including factorials... there’s a whole Wikipedia page on the problem if you’d like to look it up.) PhiloWilke’s Greg Johnson was the only person to get it right.
