Check your intuition: The birthday problem - David Knuffke

My wife’s birthday is in Junetember

OK so my girlfriend works in a primary school and can check the students birthdays, out of 551 students only 2 had the same bd yesterday, 3 had the same today and only 1 student has his birthday tomorrow. So the way it's done in this video has to be wrong

not true

Now take into account the actual probability of birthdays, e.g highest percentage of people are born in September etc.

That's too much math though.

Where's my 9/11 babies at? 😃

The animation is out of the world.

person k and i share a birthday :)

I haven’t met a single person with the same birthdate, let along in a classroom.

I have the same birthday, same day and month as a famous musician in my country, I intially thought that it was a coincidence and was quite shocked about it, while my friends were so surprised that when I answered my birthday, they immediately talked to each other. I didn’t know what they were talking about

Boa

In my class there are 5 people with a birthday on may 4th

I’m in a class with 6 students total and one of them has the same birthday as me so…

Math IA loading...

this is true tho, in my class with 32 students we have 2 people that born on the same day, month, and year!

Okayy my birthday is on november 27th lets see how many of u have it

My initial guess would have been around the square root of 365, which isn't far off at 19.

I got two classmates who have the same birthday but one left.

The sound effects are pretty annoying

But this accounts for all cases other than any two sharing their birthday. Like a group of 4 (in 23) sharing 2 different birthdays or 8 sharing a same birthday and so on. If you calculate for JUST 2 in the entire group sharing a birthday and all else having different, the probability for that would come out to be around 36% (which is also pretty high).

In my first psychology class in college, the professor used this as a way to show that people’s intuition and "common sense" is often wrong. While it’s really a mathematical puzzle, the fact that most people are wrong about this gives us insight into psychology.

The class had about 150 students in it. The professor had each of us guess how many people he’d have to call on to find two who had the same birthday. We all wrote down our answers, then he had us sit in sections of the lecture hall based on the number we’d chosen. Most people selected a number greater than 150, reasoning that you’d have to go through birthdates for half the year before you were likely to find a match.

He then started asking each student what their birthdate was. He’d write the date on the chalkboard. We reached the first match at like the 12th person he called on. He said that we’d throw that result out since it was just coincidental that we’d found a match so quickly. He kept going. The next match happened on about person number 21. He kept going, wanting to show us where we’d reach a point where almost every new birthdate was a match. We got there by person number 60-something. By the time we were into the 70s, pretty much every new birthdate matched someone else’s.

He explained the mathematics behind it, yet there were still people in the class that said it was impossible to have so many matches in a group that small. He asked them to explain why it worked, though, since we’d all seen it with our own eyes. They said it just defied common sense. The professor said that this is why "book smarts" almost always trumps "common sense." Most people simply can’t comprehend that.

Uhhh I said 27

well can i ask them directly bout their B'day ???

The choco calculator got the answer: -49851

This might have high chances on paper, bu very less in real life.
Well, still I share my birthday with a few people.

I understand the maths of it but... it just doesnt seem right

My brain hurts

lots of times I realise how common birthday matches are when they seem unlikely

And what are the odds of a person sharing the exact same birthday, day to year, to two of their classmates, who happen to be identical twins? Because it happened to me.

This problem inspired me to make a whole Java program that makes runs these simulations so I don’t have to guess or even do much math really. Using just the law of large numbers and a lot of computing power, I can let the computer do all the work.

Great video but I had to laugh at the 700 people part. Obviously it’s over 99.9% it’s literally impossible for there to be no shared birthdays in that group🤣

extremely irritating visuals

Okay, so this is just same day and month. I was thinking day, month, and year at first. Oddly, I've only known one person in my whole lifetime who had the same birthday as me...I hated her guts.

The only person i share my birthday with (not counting Ronald Weasley since he's fictional) is Justin Beiber. I've never heard his songs too
And knowing around a 100-200 people or so, with none of them sharing the same birthday, i can see why our intuition can be wrong, sort of.

He said even odds, I was blown

Actually, your wrong

me: I think around 20 people
them: 23 people, why is our intuition so wrong?
...

I missunderstood the prompt about "every birthday being equaly likely". I thought he meant that every bithday was equally represented. That lead me to a completely wrong math problem.

I have been in five different classes and no one so far has shared a birthday

I can believe this. I know couple of people who has same birthday as me, such as my 7th - 9th grade school English teacher, my fellow student at college (we have exactly same birthday, and our class had about 70 students), the children of my really close friend (we have been friends since our childhood) and the children of coworker.

the editing and sound effects were so annoying. hard to watch

if you are able to listen to a person who hopes to convince you that someone winning the lottery twice isn’t unlikely then you have no life of your own

if there are 23 people at a party they all have different birthdays then it is more likely than not you share a birthday with one of them this is absurd