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Old 12-31-2006, 02:50 AM   #1
daveB
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Default Odds of father and son having same birthday?

OT Math Geeks and stoners alike.
I'm curious:

My father and I have the same birthday 30 years apart. What are the odds?

Show you work, credit given to partial answers.
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Old 12-31-2006, 02:54 AM   #2
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1 in 365
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Old 12-31-2006, 02:55 AM   #3
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too many variables.

If his mother was impregnated on Jan 1st then he would be born roughly 9 months and a week so if your mother was impregnated on Jan 1st then the odds greatly increase but it all depends on the woman and since his mother and your mother do not share any genetic traits(which does not necesssarily help in determining how long a baby will be carried) I think it's just luck of the draw but the odds do increase with both women getting pregnant in the same time frame( 2 weeks)
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Old 12-31-2006, 02:59 AM   #4
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My algaruthm(sp?) was roughly 18 hours off.
Father = 24
Me = 25
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Old 12-31-2006, 03:01 AM   #5
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1 in 365 if all the parents just regularly tried.

Unless you picked a date (Jan. 5th) and both sets went for the same date, that would be a much lower probability.

- Stime187 does not think he's right.
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Old 12-31-2006, 03:02 AM   #6
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if it confuses you anymore: I was a preme I don't think my father was.
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Old 12-31-2006, 03:13 AM   #7
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Quote:
Originally Posted by TarmacRally View Post
1 in 365
I'd be interested in seeing people's logic to come up with anything besides this answer AND be correct.

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Old 12-31-2006, 03:31 AM   #8
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Quote:
Originally Posted by Star*Child View Post
I'd be interested in seeing people's logic to come up with anything besides this answer AND be correct.
You could certainly lower the odds by trying.

9 Months gestation, +2 weeks/-4 weeks covers about 99% of the variance.
So if you tried for, say, January 1st birthday and GOT PREGNANT on April 1st, you could have close to a 1:41.58 chance of hitting your target date. Of course the trick is getting pregnant on the right date. Assuming the female is ovulating on April 1 and you **** like crazy you could conceivably conceive on cue...

That's a best case scenario.
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Old 12-31-2006, 03:36 AM   #9
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Quote:
Originally Posted by somebody else View Post
You could certainly lower the odds by trying.

9 Months gestation, +2 weeks/-4 weeks covers about 99% of the variance.
So if you tried for, say, January 1st birthday and GOT PREGNANT on April 1st, you could have close to a 1:41.58 chance of hitting your target date. Of course the trick is getting pregnant on the right date. Assuming the female is ovulating on April 1 and you **** like crazy you could conceivably conceive on cue...

That's a best case scenario.
Those types of odds would totally depend on when the woman is fertile and women aren't fertile for a very long period of time. If you were to try, the odds could certainly be lowered, but a miss is a miss...whether it missed by a day or a few hours. The chance factor with gestation periods, ovulation, fertilization and everything working out...you might as well be talking 1:365 anyways.

The odds certainly aren't any worse than that anyways

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Old 12-31-2006, 03:40 AM   #10
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Quote:
Originally Posted by Star*Child View Post
Those types of odds would totally depend on when the woman is fertile
That would be covered where I said "Assuming the female is ovulating", I'm thinking.



The woman being fertile is a given, otherwise the odds shoot to Jesus levels.
1 : (every_other_child_ever_born - 1)
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Old 12-31-2006, 04:42 AM   #11
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I say 1 in 365 as well.

Here's one for you... I was born on March 22nd, my wife on February 6th of the same year. She was born two weeks early and I was a full month late (poor mom!)...

Feb 6th + 14 days = Feb 20
March 22nd - 30 days = Feb 20

So in all likelihood, we were conceived within hours of each other despite a 6 week "age" difference!

P03w
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Old 12-31-2006, 06:08 AM   #12
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Father is born on a set date. Odds of son being born on the same date is dependent both on:
1.) 1/365.25 (leap year bishes) days of the year
2.) It being a boy (because it can be a girl too!)...(...suckas!) 1/2

1/(730.5) chance

PS: Before people try and bring in "what if he has more than one son!" the question seems to imply (at least to me) that we're comparing "odds of father and son having the same birthday" both singular. If you want to do calculations with the average 2.47 kids per household or whatever it is be my guest .

Last edited by Petrus; 12-31-2006 at 06:13 AM.
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Old 12-31-2006, 08:45 AM   #13
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Wow, OT is the suckorz at probability. If you flip a coin twice, what is the probability that it will be heads both times? 1/2? No, it's 1/2 * 1/2 = 1/4 (can have HT, TH, TT, HH). Similarly, the probability that two people (any two people) share the same birthday is 1/365 * 1/365 = 1/133225.
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Old 12-31-2006, 09:00 AM   #14
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elucas, I had the same calculation earlier but it didn't seem correct.

365 x 365 = 133,225
but since it is essentially the same as flipping two coins wouldn't it be
1/66,612.5 or 2/133,225?


I was thinking more in terms of 2 die with 365 sides(to make it easier in my head) but I think thats where I may have screwed up because then I got 365 positive results / 133,225 combinations which would end up giving you 1/365 which didn't make sense to me
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Old 12-31-2006, 09:05 AM   #15
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Quote:
Originally Posted by daveB View Post
OT Math Geeks and stoners alike.
I'm curious:

My father and I have the same birthday 30 years apart. What are the odds?

Show you work, credit given to partial answers.
Well, I'm leaning towards teh 1/365 answer, because my dad and I are the same day too, 27 years apart (12/28)
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Old 12-31-2006, 09:14 AM   #16
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Induced labor?
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Old 12-31-2006, 09:28 AM   #17
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What if somebody offered to bet that at least two people in your math class had the same birthday? Would you take the bet?


This question is more complicated than flipping a coin, because the chance of finding two people with the same birthday depends on the number of people you ask. If there were only one other person in your math class, you might be surprised to find out that she had the same birthday as you. If there were a pair of people with the same birthday in a class of 366 people, would you still be surprised?

How large must a class be to make the probability of finding two people with the same birthday at least 50%?

Let's forget about leap year when we solve this problem (no February 29 birthdays!) This way, we can assume that a year is always 365 days long.

Also, let's assume that a person has an equal chance of being born on any day of the year, even though some birthdays may be slightly more likely than others. That will simpify the math, without changing the result signficantly.

We'll start by figuring out the probability that two people have the same birthday.

The first person can have any birthday. That gives him 365 possible birthdays out of 365 days, so the probability of the first person having the "right" birthday is 365/365, or 100%.

The chance that the second person has the same birthday is 1/365. To find the probability that both people have this birthday, we have to multiply their separate probabilities. (365/365) * (1/365) = 1/365, or about 0.27%.

Now, what about three people?

The chance of the first and second person sharing a birthday is still 1/365. The first and third person might share a birthday instead. The probability of that is 1/365 as well. But what if the second and third person shared a birthday? And what if all three of them had the same birthday?

Things are getting complicated fast. Four or five people would be even messier. Is there a simpler way?

To solve the birthday problem, we need to use one of the basic rules of probability: the sum of the probability that an event will happen and the probability that the event won't happen is always 1. (In other words, the chance that anything might or might not happen is always 100%.) If we can work out the probability that no two people will have the same birthday, we can use this rule to find the probability that two people will share a birthday:

P(event happens) + P(event doesn't happen) = 1
P(two people share birthday) + P(no two people share birthday) = 1
P(two people share birthday) = 1 - P(no two people share birthday).

So, what is the probability that no two people will share a birthday?

Again, the first person can have any birthday. The second person's birthday has to be different. There are 364 different days to choose from, so the chance that two people have different birthdays is 364/365. That leaves 363 birthdays out of 365 open for the third person.

To find the probability that both the second person and the third person will have different birthdays, we have to multiply:

(365/365) * (364/365) * (363/365) = 132 132/133 225,
which is about 99.18%.

If we want to know the probability that four people will all have different birthdays, we multiply again:

(364/365) * (363/365) * (362/365) = 47 831 784/ 48 627 125,
or about 98.36%.

We can keep on going the same way as long as we want. A formula for the probability that n people have different birthdays is

((365-1)/365) * ((365-2)/365) * ((365-3)/365) * . . . * ((365-n+1)/365).

If you know permutation notation, you can write this formula as

(365_P_n)/(365^n).

That's the same as

365! / ((365-n)! * 365^n).


We've made some progress, but we still haven't answered the original question: how large must a class be to make the probability of finding two people with the same birthday at least 50%?

We know that the probability of finding at least two people with the same birthday is 1 minus the probability that everybody has a different birthday, and we know how to find the probability that everybody has a different birthday for any number of people. The easiest way to find the right class size is to use a calculator to try different numbers in the formula. It turns out that the smallest class where the chance of finding two people with the same birthday is more than 50% is... a class of 23 people. (The probability is about 50.73%.)

http://mathforum.org/dr.math/faq/faq.birthdayprob.html
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Old 12-31-2006, 09:35 AM   #18
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http://www.suburbanchicagonews.com/n...DAY_S1.article

And baby makes three
Naperville siblings share the same birthday ... but no, they're not triplets

December 26, 2006
BY KATIE FOUTZ Staff Writer

Nine-year-old Ben Lueken already thought it was exciting to share a birthday with his younger brother.

Matthew, 6, was born Dec. 5, 2000, three years to the day after Ben was born. Since then, when their interests aligned, they have held joint birthday parties and shared birthday cakes.

Then their parents, Linda and John Lueken of Naperville, learned they were expecting a daughter. Her due date: Dec. 5, 2006.

"Both me and my brother thought that (her birthday) would be on Dec. 5," Ben said. "But our mom and dad didn't. When it actually came, and they called our grandma from the hospital, we were jumping up and down on our beds."

Beating 48 million-to-1 odds - John checked with two statisticians - Gabrielle Lueken arrived naturally at 6:27 a.m. Dec. 5. Her two big brothers chose a stuffed cat and a doll-and-blanket combination as presents for her.

"Our baby sister was a present," Matthew said.

The Luekens insist this happened by pure luck. In 1997, Ben came a week late, and labor had to be induced. In 2000, Matthew came 10 days early but entirely on his own.

On Dec. 4 this year, several people called Linda, a stay-at-home mom, to see if she was in labor.

"'So, are you going to have that baby tomorrow?'" Linda said they'd ask. "'Or are you going to wait?' I said, 'Well, she's going to wait! I have nothing to do with it.'"

John, the director of construction for McDonald's Corp., was leaving a leadership meeting Dec. 4 when he and his colleagues discussed the odds.

"Most of the response has been, 'You need to go to Vegas,'" he said. "That, or play the lottery numbers."

Linda has never heard of another family having three kids with the same birthday without being triplets. However, two cousins from Aurora and Montgomery were born on the same day - Aug. 26 - this year at Rush-Copley Medical Center in Aurora.

In the entertainment world, Britney Spears and Kevin Federline almost had two same-birthday children - their son, Jayden James, was born Sept. 12, two days shy of their son Sean Preston's first birthday. It did happen to actor Jeffrey Tambor, former star of "Arrested Development" - his daughter, Eve, was born Dec. 10, the same birthday as his 2-year-old son, Gabriel.

Linda said friends and family can't believe her children's triple birthday is even possible.

"They're just blown away," she said. "I believe it was just meant to be."

The boys are looking forward to sharing birthday parties with their new sister. They might even make some concessions about the theme.

"You think you'll have all planes and trains?" John asked.

"No, let's do some Barbie dolls," Matthew replied.
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Old 12-31-2006, 11:05 AM   #19
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My father-in-law and brother-in-law share the same birthday.
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Old 12-31-2006, 11:21 AM   #20
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Quote:
Originally Posted by ScubaruImpreza05 View Post
My algaruthm(sp?) was roughly 18 hours off.
Father = 24
Me = 25
what the ****!?
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Old 12-31-2006, 11:25 AM   #21
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Quote:
Originally Posted by Neek View Post
Beating 48 million-to-1 odds - John checked with two statisticians - Gabrielle Lueken arrived naturally at 6:27 a.m. Dec. 5. Her two big brothers chose a stuffed cat and a doll-and-blanket combination as presents for her.
That's BS - its only 48 million-to-1 (1/365^3 = 1/48,627,125) if they picked Dec 5 BEFORE the first child was born (I bet they asked the statistician "what are the odds of 3 babies all being born on December 5?", not "3 babies being born on the same day?")

As it is, Dec 5 is only significant because it was the day the first child was born. The other two are the coincidence... so its (1/365^2 = 1/133,225)

and, of course, all of the other variables people have mentioned... if they were trying for Dec 5 (which they probably were, after the first two were born) the odds are much better.
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Old 12-31-2006, 11:26 AM   #22
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My two cousins (same family) have the same birthday either a year or two years apart.

My sister has the same birthday (same day) as her twin cousins, who's mother is my mothers sister. All born on December 18th, 1989. Same hospital, same room.

Other oddities, the majority of my grandmothers grand children are born in the fall (most in October). There are 22 (including me) of them and I think only 3 are not fall birthdays.

One more that I can think of is that myself and many of my friends are born on the 11th of a month. Every 11th we call around to see who it is. There are 8 of us who have this, none with the same birthday.
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Old 12-31-2006, 11:29 AM   #23
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Quote:
Originally Posted by elucas730 View Post
Wow, OT is the suckorz at probability. If you flip a coin twice, what is the probability that it will be heads both times? 1/2? No, it's 1/2 * 1/2 = 1/4 (can have HT, TH, TT, HH). Similarly, the probability that two people (any two people) share the same birthday is 1/365 * 1/365 = 1/133225.
you are correct sir ha ha!
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Old 12-31-2006, 11:31 AM   #24
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thats pretty cool my mom and i have the same birthday like 15 years apart.
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Old 12-31-2006, 11:34 AM   #25
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Hm.

My dad and his dad have the same birthday.

My sister and our grandmother have the same birthday.
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