[link|http://www.maa.org/mathland/mathtrek_11_23_98.html|Birthday Surprises]
It's possible to derive a simple formula that gives an approximate answer for the number required to get a 50-percent chance of a match. That number is 1.2 times the square root of the number of categories. For 365 categories (days of the year), the number is 1.2 multiplied by the square root of 365, or 23.
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What about triple matches? That's a trickier calculation, but the answer turns about to be 88 people.
Cheers,
Scott.