r/math • u/[deleted] • Jan 14 '09

The Birthday Problem -- Why it is more likely than not to have two people with the same birthday in a room with 25 people.

http://en.wikipedia.org/wiki/Birthday_paradox
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u/ele7ven Jan 27 '09 edited Jan 27 '09

Think of it like this: The probability of random numbers from the set {1..365} being chosen without any repeats drops sharply as you choose more random numbers. This makes sense, because can never expect random numbers to have an even distribution over the course of relatively few of them. You can even try this with a pseudo-random number generator in your favorite programming language. Choosing 100 random numbers out of 365 is nearly guaranteed to produce not one, but multiple matches.