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| Fynbos of the Western Cape, South Africa |
Birthday Parties
It is a fairly well-known fact that in any group of 23
people, there is at least a 50-50 chance that two of them share a birthday.
In a group of 5, the chance that two have the same birthday is
just under 3 in 100; for 15 it climbs to just over 1 in 4; and for 23 it is
nearly 1 in 2.
The reason lies in a quirk of statistics. As the size of a
group increases, the number of possible pairs increases as well – but at a
faster rate. In group 5, there are 10 possible combinations of 2 people; in a
group of 23 there are 253 possible pairs.
In his book Lady Luck. The mathematician Warren Weaver
relates how this curios fact came up in conversation during a dinner party for
a number of army officers during World War II.
Most of Weaver’s fellow
guests thought it incredible that the figure was just 23; they were certain it
would have to be in the hundreds. When someone pointed out that there were 22
people seated around the table, he put the theory to the test.
In turn, each of the guests revealed his birth date, but no
two turned out to be the same. Then the waitress spoke up. “Excuse me,” she
said. “I am the 23rd person in the room, and my birthday is May 17,
just like the general’s over there.”
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