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The Sciences

Fuzzy Math

In a group of 23 people, the odds are better than even that at least two of them will share a birthday.

By Alex StoneMay 28, 2006 5:00 AM

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In a group of 23 people, the odds arebetter than even that at least two of them will share a birthday.

Seems as if you'd need many more people before you'd find a shared birthday, doesn't it? But consider the reverse situation, in which nobody shares a birthday. Since all birth dates are equally likely, then the odds of picking 23 different ones out of 365 aren't that good—less than fifty-fifty. Among 200 people, there is a 99.9999999999999999999999999 percent chance that two birthdays will match! So why don't you meet more people who share your birthday? That's because a specific match is less likely than an arbitrary one. For any two people to share a birthday, any day will suffice; for someone to share your birthday, only one does.

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