Another Birthday Coincidence

[Sleepygirl]

My birthday is on the 23rd of January . My sons Birthday is 2nd of August. Only one of the people that I know of that shares my birthday has a child born the 2nd of August also. What are the odds of this happening. Is this a common occurence or not?

[Guitarmarama]

It's quite obviously a coincidence. However it depends on what you class as being a coincidence. There's 365.25 days in a year, so theres a 1/365 chance that you'll meet someone with your birthday. Regardless of background, the child also has a 1/365 chance of being born on that day. The only time you can predict it, is if both you and your friend made sure you concieved around the same date. Even so, as this is not the case (I don't think!), then to be honest, I think the chances are simply 1/365 x 1/365, whatever that is! Obviously the fact that it's mother/child will make this smaller, but I'm sure it's not THAT rare in comparison to the fact that there are seven and god knows how many billion people on the planet. My auntie's 27th birthday was the very day that Elvis Presley died (One of her favourite artists), and my mum's 27th birthday was the day that John Lennon died (One of her favourite artists), and my mum and auntie are sisters - Now THAT'S freaky! But yeah, yours is as freaky as mine!

[coggles]

Guitarmarama, that means your auntie is 3 years younger than my mum, who was 30 on the day Elvis died - 16.08.77, also a big Elvis fan.

[Greedyfly]

My mums is on the 23rd January

[sft42]

I was once in a band where all three members partners had the same birthdays (January 24th) and they were seperated by a few years between them......odds on that one anyone?

[bernardo]

c. 1 in 130000

[Kerplunk]

Isn't it more like 1 in 48727112.203125 ?

[Sleepygirl]

Bernado and Kerplunk are your odds for my question or for sft42's?

[Guitarmarama]

Or mine???

[tomd]

In reality you couldn't take someone being your friend and the date on which their birthday falls as being independent (ie. one could affect the other). Especially with the band, after realising they all had the same birthday they could have decided to make something of it, or even gone searching for a third member with the specific birthday. Time of year is also not independant of rate of birth, for obvious reasons birth rates swell 9 months after christmas & new year holidays (How many people do you know born in September compared to January?). In addition to this, these two figures only work out the probably that a specific friend chosen has this conincidence, ie. you probably have lots of friends not born on your birthday, so the more friends you have, the more likely it is. Not a straightforward problem, unfortunately. That set aside, the odds on yours would be 1 in 365.25^4, and stf's 1in 365.25^3, if it were that straightforward.

[bernardo]

Sleepygirl - it's the same thing, i.e. the probability of two people having the same birthday (1 in 365) multiplied by the probability of two other people having the sme birthday (1 in 365).

Kerplunk - no, it's 1 in 130thousandish. i.e. the probability of three randomly chosen people having the same birthday. The prob of person A having the same bday as person B is 1 in 365; the prob of person C having the same bday as A and B is 1 in 365; which makes 1 in 130,000ish. The figure of 1 in 48,000,000ish is only if you specify a particular date before the first person appears. The prob of 3 randomly chosen people having their bdays on 24th January is 1 in 48,000,000ish, but in the example given above it would have been equally remarkable if they had had their bdays on (e.g.) 13th September. So the coincidence is 1 in 130,000ish for a non-specific date.

[Sleepygirl]

Thank you Bernardo. I was making it far more complicated. I thought maybe it was 1/365 multiplied by 4 to take into the account the odds in each family. I wasn't sure about the math at all which was why I asked.