"The great control freak, math"

What are the odds that two people in a group of 40 have the same birthday? In an amusing post Alan Bellows describes his nemesis, math, and the Birthday Paradox, an odd statistical principle.

It's tricky to explain the phenomenon in a way that feels intuitive.You can consider the fact that forty people can be paired up in 780 unique ways, and it follows that there would be a good chance that at least one of those pairs would share a birthday. But that doesn't really satisfy the question for me, it just feels marginally less screwy. So I did something quite out of character: I crunched the numbers.

The answer, if the birthdays were evenly distributed (they're not of course), would be about 90 percent.