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# Same birthday, different year, SAME FAMILY Probability

Sat 5 Jun, 2004 10:20 am
Hi All
I'm doing some research for a project I'm working on
and I am looking for insight.

I have seen the probability equations for 2 people having the same birthday but does anyone know the probability of 2 or more people in the same immediate family (such as siblings) having the same birthday but different years.

I have 4 people in my family with the same birthday but different years.
2 of my kids (2 years apart)
my father
my grand father

all on feb 26

Is there a NAME for people of the same family born on the same day but different years
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Type: Discussion • Score: 23 • Views: 39,606 • Replies: 43
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jespah

1
Sun 6 Jun, 2004 08:31 am
Hmm I'd be interested, too. My nephew and I are born on the same date. My cousin and her nephew are also born on the same date (a different date from my and my nephew's birthday). And, another nephew of mine is going to born, on the other side of the family. He's due in the right month (September), so there's a possibility that I will have two nephews, one from my side and one from my husband's side, both born on my birthday.

What are the odds?

PS Welcome to A2k!
0 Replies

billy falcon

1
Tue 8 Jun, 2004 07:44 pm
Abear, they're called "statistial probabilities."

The puzzler you are looking for is "How many people do you need to be absolutely certain that at least two people share the same bithday? Since there are 366 days in a year, you would need 367 people.

Now, if you want to be 50% certain that two people in group shared the same birthday? You might guess about half of 365 (the days in a year) or 183.

The anwer is, there needs to be only 23 people.
Half of the time that twenty-three randomly selected people are gathered together, two or more will share a birthday.

The statistical explanation is beyond me to explain.
However, I find it helpful to be aware that if you put 23 people in a circle, the first person is paired with 22 people and leaves. The next person is paired with 21 poeple and leaves. The next perons is paired with 20 persons and leaves. And so on. You can see that there are many more pairs of people than 23.

So, the odds are quite high that in a classroom, meeting, extended fmily, bowling group, etc. you're
very likely to get two or more people sharing a birthday.

i hope this helps.
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Mr Stillwater

1
Tue 8 Jun, 2004 08:28 pm
Cousins.

I share the same birthday with a blood relation (different years though).
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Shekeda

1
Tue 8 Jun, 2004 08:53 pm
I shared a birthday with my paternal grandmother (born late 19th century) and my ex-husband's father was also born on that date (July 9). My sister's birthday is July 5 - so she almost made it, if only Mum could have held on a few days longer

Interestingly, after Mum died my Dad married again to a woman who had a daughter born July 7 - right between my sister and I!

<-- Just noticed the coincidence. I joined A2K on my eldest son's (33rd) birthday
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Mr Stillwater

1
Tue 8 Jun, 2004 09:04 pm
Welcome to A2K Shekeda! Always good to see another Aussie online!
0 Replies

Shekeda

1
Wed 9 Jun, 2004 06:00 pm
Thanks Mr Stillwater - but as you can see, I'm a Pom!

LOVE A2K - much better than the other Aust site with a forum that I belong to!
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fachatta

1
Wed 16 Jun, 2004 02:35 pm
Probability = Number of Combinations we're looking for / Total number of combinations of birthdays.

If N = number of people, the number of total possible birthday combinations they can have is 365^N (no leap years)

the number of ways N people can all NOT have the same birthday = 365 * 364 * 363 * (365 - n + 1)

so the probability no two people have the same birthday is

365 * 364 * ..... (365 - n + 1) / 365 ^ n

The probability two people do have the same birthday is 1 - the previous equation. with 23 people , the probability is 50.73%
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cjhsa

1
Wed 16 Jun, 2004 03:40 pm
Last summer, a girl showed up at the pool with the same hair color, the same name, the same birthday (two years younger) and even the same bathing suit as my daughter. They had never met before. What is the probability of that?
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patiodog

1
Wed 16 Jun, 2004 03:44 pm
There you have it: my birthday is also July 5 (like Shekeda's birthday). What are the odds? Judging by the number of people I've met who share my birthday, pretty good.
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fbaezer

1
Wed 16 Jun, 2004 03:45 pm
cjhsa wrote:
Last summer, a girl showed up at the pool with the same hair color, the same name, the same birthday (two years younger) and even the same bathing suit as my daughter. They had never met before. What is the probability of that?

Zero in Afghanistan (what would be the fate of an Afghan girl in a bathing suit?!).
Minimal in USA - 2003.
Much larger in China during the Cultural Revolution (millions of black haired girls named Li and only one color for bathing suits)
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mrflashpot

1
Fri 4 Feb, 2005 04:32 am
Same birthday
I am the father of four daughters. Three of which were born on March 19. One in 1984 one in 1986 and one in 1989
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mrflashpot

1
Fri 4 Feb, 2005 04:34 am
I am the father of four daughters. Three have the same birthday March 19. one in 1984 one 1986 and one 1989. All were natural child birth
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FreeDuck

1
Fri 4 Feb, 2005 06:48 am
My sister-in-law has three daughters, all of them born on January 1st, three years apart.

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engineer

1
Fri 4 Feb, 2005 09:29 am
December is better than January.
FreeDuck wrote:
My sister-in-law has three daughters, all of them born on January 1st, three years apart.

Think about all the lost tax breaks! Just one day earlier!
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engineer

1
Fri 4 Feb, 2005 10:12 am
Insight
Quote:
I'm doing some research for a project I'm working on
and I am looking for insight.

OK, back to trying to help. Let's make some assumptions about families. Let's say the group of people you are considering has 8 people (parents, two children, two sets of grandparents). The probability of two having the same birthday (as shown in an earlier post) is:

1 - 365! / (365-8)! / 365^8 = 7.3%

That means that you would expect one out of every 14 families on average to have common birthdays. You should be able to go around the room in your class and find people who have common birthdays in their families. That doesn't mean that they will share that common birthday, only that there is a shared on in their family. If you extend the group out to aunts, uncles, cousins, etc. there will be lots of matches. Between 10 and 30 people, the probability of a match goes up by around 3% per person added. As noted earlier, 23 people is around the 50% point. If we assume the typically family has two children, then grandparents (4), uncles and aunts (4), cousins (4), parents (2), sibling (1) and you add up to 20 and a 41% common birthday probability. Basically, you aren't going to find any lack of families with births in common.
0 Replies

1
Fri 4 Feb, 2005 11:58 am
FreeDuck wrote:
My sister-in-law has three daughters, all of them born on January 1st, three years apart.

Is your brother's birthday on March 14th? Sound's like he's gettin' lucky!
0 Replies

FreeDuck

1
Fri 4 Feb, 2005 12:51 pm
FreeDuck wrote:
My sister-in-law has three daughters, all of them born on January 1st, three years apart.

Is your brother's birthday on March 14th? Sound's like he's gettin' lucky!

Ha ha. Could very well be.

engineer wrote:

Think about all the lost tax breaks! Just one day earlier!

Hah. They don't live in the US so luckily that wasn't a concern. My dad, however, was thrilled that my two sisters were born the same year (one in January the other in December) because he got a double tax break!
0 Replies

Leftrightchicken

1
Wed 12 May, 2010 07:09 am
@Abear,
HI I have a The same Birthday as both of my paternal parents I was born on june 26th and they were born 30 years prior on june 26th just thought i would throw that at you to think about
0 Replies

LUSK04

0
Thu 13 Jan, 2011 02:08 pm
@Abear,
OK, I NEED HELP!!! ME, MY SISTER AND MY BROTHER ALL HAVE KIDS THAT HAVE THE SAME BIRTHDAY BUT DIFFERENT YEARS. WE HAVE A BIRTHDAY PARTY FOR THE 3 OF THEM, AND I AM IN CHARGE OF GIFT BAGS. INSTEAD I WOULD LIKE TO GET A TSHIRT MADE FOR THEM AND THE OTHER COUSINS ATTENDING THAT REFERS TO SOMETHING ABOUT THE COUSINS THAT ARE SHARING THE SAME BIRTHDAY, BUT THAT THE OTHER COUSINS COULD WEAR TOO!!! SORRY IF CONFUSING. HELP IDEAS GREATLY APPRECIATED!!

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