Hi Annette-
The general formula was postulated by Percy Diaconsis (sp?) at MIT. To
achieve a 90% probability of two events happening it is: 1.6 times the
square root of the contingencies. The aproximate contingency of a
birthday (as opposed to birthdate) is 1/365 (if we ignore leap year)
therefore 1.6 times the square root of 365 = 30.57. This means that if
you have 31 students in your class there is a 90% chance that two of
them will share a birthday. I often use this as a powerful demonstration
of how people often underestimate the odds of things occuring -
especially when discussing ESP. For smaller classes it is better to have
students randomly pick a number from 1 -100. There the 90% probability
of a match would occur with only 16 participants. If you only wanted to
have a 50% certainty of a match you would only need 1.2*Sqrt C or 12
people. I'm afraid that I don't have a reference for this as I simply
remember it from a lecture that he gave a number of years ago. He did
not provide a proof at the time & I'm not sure that I could have
followed it if he did.
Hope that helps,
-Don.
[EMAIL PROTECTED] wrote:
Can someone tell me the formula for calculating the probability of two students
in a class having the same exact birthdate?
What about THREE in a class of 20?
Thanks
Annette
Annette Kujawski Taylor, Ph.D.
Professor of Psychology
University of San Diego
5998 Alcala Park
San Diego, CA 92110
619-260-4006
[EMAIL PROTECTED]
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Don Allen
Department of Psychology
Langara College
Vancouver, B.C., Canada
V5Y 2Z6
604-323-5871
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