Oh my god, that's my birthday, too.
That's amazing.
Not really, though.
Hi, I'm Ben.
And today's question is, what's going on
with the birthday paradox?
You've probably heard this one before.
The idea that if there are 20 people in a room,
there's a 50/50 chance that two of them
will have the same birthday.
So how can this be?
It really is called the birthday paradox,
and it turns out it's useful in several different areas,
for example, in cartography and hashing algorithms.
You can try it yourself.
The next time you're at a gathering of 20 to 30 people,
ask everyone for their birthday.
I mean, don't be creepy about it.
Play it cool.
Say I'm trying to prove the thing for science, or whatever.
And it is likely that two people in this group
will have the same birthday-- not
around the same time, the exact same day.
This really surprises people.
So the reason it's so surprising is
because we're used to comparing our particular birthdays
with some other individual's particular birthday.
So for example, you meet somebody randomly
and you ask her what her birthday is.
The chance of the two of you having the same birthday
is only one out of 365, or 4.27%.
In other words, the probability of any two individuals
having the same birthday is low.
Even if you asked 20 individual people,
the probability is still low, it's less than 5%.
It's natural that we feel like it's
very rare to meet anybody who has
the same birthday as our own.
When you put 20 people in a room,
however, the thing that changes is the fact
that each of these 20 people is now asking each of the other 19
people about their birthday simultaneously.
Each individual person only has a small chance,
less than a 5% chance of success.
But everyone's trying it at the same time,
and that increases the probability dramatically.
So the next time you're with a group of 20 or 30 people,
why not give it a try?
You might be surprised.
So that's it.
Thanks so much for watching.
I hope you enjoyed this.
If you are feeling in a good mood,
or you want to learn more about people's birthdays,
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