Author: Rémi Coulom
Date: 04:55:56 02/18/99
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On February 18, 1999 at 07:46:53, Kai Lübke wrote: >On February 18, 1999 at 07:19:43, Jouni Uski wrote: > >>If You select random 23 people, it's over 50% probability that two have exact >>same birth day (from 365 possibility)! This seems to be against common sense. > >Depends on how you look at it. If you take one of them fixed, then the >probability that one of the other 22 has the same birthday is of course 22/365, >less than 10%. >But the case here of course is the question whether *any* of them have the same >birthday. >So you have a probability of 22/365 for the first person's birthday to conincide >with any of the others, for the second person the prob. is 21/365 (as we need >not compare with #1 again), for the third 20/365 and so on. >So the overall probability is (22+21+...+2+1)/365 = 253/365 >50%. > >--- >Shep Your formula is wrong. It would give a probability of 18200% for 364 persons! The exact formula is 1 - q!/((q-p)!*q^p) for p choices among q values, p < q. Remi
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