Probability of three generation being having the same birthday

[adrenalinxx]

What is the probability of a Grandfather his son and then his son (Grandfather-father-son) having the same birth day.

[bernardo]

1 over 365 squared, which is 1 over 123,225

[bernardo]

That was a misprint: It is 133,225

[j2buttonsw]

I thought I read (perhaps even on here) that it you will meet someone with the same birthday as you in a room of only 30-40 people (well, significantly lower than the 365 which you would expect). Or am I confusing it with something else?

[tigga]

Slightly different, but i recall a story in a newspaper about a pair of twins who both gave birth to twins on their own 20th or 30th birthday.

When you consider the amount of people in the world, i guess these coinsidences have quite a good chance of happening to somebody !.

[kempie]

What about Leap Years?

Does this make the equation 365.25 squared?

& what if one of them is born on 29th Feb... how would that affect the equation?

 

[Bangkok]

the odds can be cut dramatically by planning, if those involved were so inclined and lucky.  133,225 is the chance of it happening randomly. 

[natalie_1982]

Not necessarily, as there is more chance of being born on certain days than others.  I remember from my biology class that the most common month to be born in is September - 9 months before September is Xmas/ New Year time and a lot of people drinking alcohol lose their inhibitions, have unprotected sex, get caught in the moment of a family atmosphere and decide to have a child, etc etc.  It's not quite that clear cut obviously, but I think it's more than just 1 in 133,225.

[bernardo]

j2buttonsw - slightly different.  If you have a room full of people, it is more likely than not that there are two (or more) people with the same birthday, if the room contains 23 or more people.  In other words, any two out of the 23.  You would need far more people in order to find a match for a specific date, such as your own birthday.

 

In my original answer I deliberately ignored leap-years to make it easier.  The probability for a day other than 29th February would be 4 over 1461 squared, or 1 over 365.25 squared.  The probability of the three people having their birthdays all on 29th February would be 1 over 1461 cubed, which would be about 1 over 3000million.

[ludwig]

My dad and grandad were both born April 6th. I was born April 4th. I keep telling my mum she should have held on for a couple more days.

[mindofnufin]

I have heard that some cultures have certain days that have religous/lucky implications for fertility.

in these cultures same day birthdays are more common.

[tigga]

natalie_1982, thaks for too much info, i was born in september and had rather not thought about it like that ! lol