Birthday paradox
Poll: Birthday paradox
Total Members Polled: 89
Discussion
I say paradox but it is more a counterintuitive result. The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday. If you do the mathematics the answer is just 23, which is smaller than most classes at school (unless you are in a private school perhaps).
I can’t recall being in a class with someone having the same birthday. From my time at school it seemed rare, whereas from the mathematics you would expect every other class to have at least two people sharing a birthday. I suppose it is possible that it isn’t entirely random in that the when the school decides which classes to put people in, they might avoid putting people in the same classroom.
Is this something you came across at school?
I can’t recall being in a class with someone having the same birthday. From my time at school it seemed rare, whereas from the mathematics you would expect every other class to have at least two people sharing a birthday. I suppose it is possible that it isn’t entirely random in that the when the school decides which classes to put people in, they might avoid putting people in the same classroom.
Is this something you came across at school?
After posting I wondered whether I could do an experiment to check the maths. I found a random number generator on line:
https://www.calculatorsoup.com/calculators/statist...
I only did four attempts…and there was at least one matching pair in three of those (one had two matching pairs!)
So despite it being counterintuitive it seems correct.
Maybe people have shared birthdays in classes without even knowing it.
https://www.calculatorsoup.com/calculators/statist...
I only did four attempts…and there was at least one matching pair in three of those (one had two matching pairs!)
So despite it being counterintuitive it seems correct.
Maybe people have shared birthdays in classes without even knowing it.
I think you'll find you're somewhat distorting the birthday paradox here.
The first and obvious point to make is that it's not how many people there need to be in a room before you are statistically more than 50% likely to share a birthday with someone - that's going to be around the 180 mark depending on what time of year you were born - but of any two people sharing a birthday.
The second less obvious one is that the school year only covers 39 weeks out of 52. I suspect that my mother might possibly know if two of my primary school classmates had the same birthday (that wasn't mine) even out of term time back in the days where everyone in the class was invited to everyone else's party, but by the time you get to secondary school, what are the chances that you're even going to know that two of your classmates' birthdays are the same if they happen outside term time?
I've no idea how you'd calculate the impact of that second point though, not least because paradoxically you're probably more likely to remember two classmates sharing a birthday if it happens to be Christmas Eve, Christmas Day, Boxing Day, New Year's Eve or New Year's Day I'd imagine.
The first and obvious point to make is that it's not how many people there need to be in a room before you are statistically more than 50% likely to share a birthday with someone - that's going to be around the 180 mark depending on what time of year you were born - but of any two people sharing a birthday.
The second less obvious one is that the school year only covers 39 weeks out of 52. I suspect that my mother might possibly know if two of my primary school classmates had the same birthday (that wasn't mine) even out of term time back in the days where everyone in the class was invited to everyone else's party, but by the time you get to secondary school, what are the chances that you're even going to know that two of your classmates' birthdays are the same if they happen outside term time?
I've no idea how you'd calculate the impact of that second point though, not least because paradoxically you're probably more likely to remember two classmates sharing a birthday if it happens to be Christmas Eve, Christmas Day, Boxing Day, New Year's Eve or New Year's Day I'd imagine.
Did you read my post? If yes your comprehension skills are pretty poor as I wrote:
“The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday.”
Where in that sentence did I mention someone sharing my birthday?
“The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday.”
Where in that sentence did I mention someone sharing my birthday?
I don't know about everyone else's school, but in my primary school there were 4 classes per year and the pupils were allocated a class based on birth dates so that most pupils were a similar age/maturity. This meant that everyone in my class had a birthday between October and December so far higher odds of birthday sharing. 55 years later I can still remember the 4 kids who shared my birthday week and the one who shared my birthday. Hi Dave Smith if you're still out there!
Stuart70 said:
People go on holiday in August, a larger number of children are born in May.
More likely to share a birthday if born in May.
Sex, that is what it is
.
It’s amazing the look on peoples faces when you ask them what significant event happened in your parents life 9 months before you were born. In my case, their wedding anniversary, in my sister’s case, their birthdays (1 week apart). The dawning realisation is great to see, and it’s very common for there to be some of ‘link’ to a significant event.More likely to share a birthday if born in May.
Sex, that is what it is
. (ETA: although I don’t want to think about it, I’m sure my parents had sex on days other than ones marking significant events)
Skeptisk said:
Did you read my post? If yes your comprehension skills are pretty poor as I wrote:
“The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday.”
Where in that sentence did I mention someone sharing my birthday?
Gosh, you're a sensitive little poppet, aren't you? “The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday.”
Where in that sentence did I mention someone sharing my birthday?
It was this bit that made me leap to my assumption.
Skeptisk said:
I can’t recall being in a class with someone having the same birthday.
Kermit power said:
Skeptisk said:
Did you read my post? If yes your comprehension skills are pretty poor as I wrote:
“The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday.”
Where in that sentence did I mention someone sharing my birthday?
Gosh, you're a sensitive little poppet, aren't you? “The original question is how many people, chosen at random, would you need to have in a room to have a more than 50% probability that at least two people share a birthday.”
Where in that sentence did I mention someone sharing my birthday?
It was this bit that made me leap to my assumption.
Skeptisk said:
I can’t recall being in a class with someone having the same birthday.
Stuart70 said:
People go on holiday in August, a larger number of children are born in May.
More likely to share a birthday if born in May.
Sex, that is what it is.
Doing the "9 months before your birthday" is quite entertaining with friends. Especially the September / October ones. Hmmm, party time?More likely to share a birthday if born in May.
Sex, that is what it is
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