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Originally Posted by HarryT
Another example is that if you take a random sample of 20 people, the odds are better than even that two of them will have the same birthday. Take 30 people, and it's virtually certain.
Statistics are only surprising to those who don't understand maths  . |
50% is at 23 persons and it seems to be around 65% for 30 according to wikipedia:
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In probability theory, the birthday problem, or birthday paradox[1] pertains to the probability that in a set of randomly chosen people some pair of them will have the same birthday. In a group of at least 23 randomly chosen people, there is more than 50% probability that some pair of them will both have been born on the same day. For 57 or more people, the probability is more than 99%, and it reaches 100% when the number of people reaches 366 (by the pigeonhole principle, ignoring leap years). The mathematics behind this problem leads to a well-known cryptographic attack called the birthday attack.
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