WEBVTT - What is the Birthday Paradox?

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<v Speaker 1>Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,

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<v Speaker 1>it's Christian saga and today's question is what is going

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<v Speaker 1>on with the birthday paradox. You've probably heard this one before,

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<v Speaker 1>the idea that if there are twenty people in a room,

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<v Speaker 1>there's a fifty fifty chance that two of them will

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<v Speaker 1>have the same birthday. So how can this be? Well,

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<v Speaker 1>it really is called the birthday paradox, and it turns

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<v Speaker 1>out it's useful in several different areas, for example, in

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<v Speaker 1>cartography and hashing algorithms. You can try it yourself the

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<v Speaker 1>next time you're at a gathering of people, you know,

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<v Speaker 1>just ask everyone for their birthday. I mean, don't be

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<v Speaker 1>creepy about it, play cool, say you know, something like

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<v Speaker 1>I'm trying to prove this for science or whatever. And

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<v Speaker 1>it's likely that two people in this group will have

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<v Speaker 1>the same birthday, not around the same time, they will

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<v Speaker 1>have the exact same day. And this really surprises people.

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<v Speaker 1>So the reason isn't so surprising. It's because we're used

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<v Speaker 1>to comparing our particular birthdays with some other individuals particular birthday. So,

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<v Speaker 1>for example, you meet somebody randomly and you ask her

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<v Speaker 1>what her birthday is, the chance of the two of

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<v Speaker 1>you having the same birthday is only one out of

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<v Speaker 1>three hundred and sixty five, or four point to seven percent.

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<v Speaker 1>In other words, the probability of any two individuals having

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<v Speaker 1>the same birthday is low. Even if you asked twenty

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<v Speaker 1>individual people, the probability is still low. It's less than

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<v Speaker 1>five percent. It's natural that we feel like it's very

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<v Speaker 1>rare to meet anybody who has the same birthday as

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<v Speaker 1>our own. But when you put twenty people in a room, however,

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<v Speaker 1>the thing that changes is the fact that each of

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<v Speaker 1>these twenty people is now asking each of the other

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<v Speaker 1>nineteen people about their birthday simultaneously. Each individual person only

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<v Speaker 1>has a small chance, less than a five percent chance

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<v Speaker 1>of success, but everyone's trying at the same time, and

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<v Speaker 1>that increases the probability dramatically. So the next time you're

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<v Speaker 1>with a group of twenty or thirty people, why not

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<v Speaker 1>give it a try. You might be surprised. Check out

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<v Speaker 1>the Brainsuff channel on YouTube, and for more on this

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<v Speaker 1>and thousands of other topics, visit how stuff works dot com.