Birthday stats

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Rigil Kent (August 11, 2009, 10:40:36 AM):
Even though there are only a few tens of people listed on my birthday calender, no less than 4 of them share my date of birth (i.e. day and month - not year). This seems an unlikely high number to me.

According to my (possibly dodgy) calculation, I would have to know the birth dates of about 365 x 4 = 1460 people to make it statistically likely to identify 4 people who unwrap gifts on the same day as I do.

How many people do you know of that share your birth date? Have you noticed the same trend, or is my case just a fluke?

Then I heard another interesting thing from a reliable source - well as far as unsolicited emails go - that if you take a class of 32 kids, the chances that at least any 2 of them share a birthdate is as high as 50% !

Mintaka
cyghost (August 11, 2009, 11:01:24 AM):
A girl I had a crush on in High School shared my birthday which was kinda cool

And today my wife and I share the same birthdate :D That makes it fairly easy.

Other than that I can't recall any sharing my birthday...
AcinonyxScepticus (August 11, 2009, 11:33:00 AM):
Then I heard another interesting thing from a reliable source - well as far as unsolicited emails go - that if you take a class of 32 kids, the chances that at least any 2 of them share a birthdate is as high as 50% !


It's actually a 50% probability when there is a sample of 23 random people, at 30 the probability increases to 70%.

I do not know anyone personally who shares my birthday. Although, Valentino Rossi was born on the same day, but that's nothing even related to this discussion because I don't actually know him. (I thought that was true based on my brother's encyclopaedic knowledge of bikes, but it turns-out that it is not even close).

But the main difference here between the probability of any random two people sharing a birthday (any birthday) and people who share your particular birthday is radically different.

The chances that I would share a birthday with someone in particular (say you, Mintaka, for example) is 1 in 365, the chance that we would both share it with a particular third person (say bluegrayV) is 1 in 133 225. A particular fourth person would be 1 in 48 627 125. This is only because I don't know your birthdays. If I do know your birthdays, and they are the same then the probability is 1 in 1. Odd, huh?

ETA: There's an experiment on this at Richard Wiseman's blog, here.

James
Mefiante (August 11, 2009, 14:12:06 PM):
Is that four people in addition to yourself, i.e. five people sharing the same birthday? If you give me the total number of birthdays on your calendar, I’ll work out the probability of four (or five) people sharing a birthday in that number.

As James has pointed out, in a group of 23 people there is a slightly better than even chance that two of them will share a birthday. This surprisingly low number is, unsurprisingly, known as the “birthday surprise” or “birthday paradox.”

'Luthon64
Faerie (August 11, 2009, 14:46:13 PM):
None in my case, however I've noticed a tendency of familial birthday's being clustered in same months. e.g my dad, brother, my eldest son and I are in April, whereas my Mother, my youngest son and second brother are in June.

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