I realize in any random sample, the odds of any two people having the same birthday is approx. 1:365.
but doesn't that go up in a non-random sampling? IE, don't you have to multiply the chances of two people being brothers-in-law by the chances of two people sharing the same birthday or something?
So what are the chances of two people sharing the same birthday AND of one marrying the sister of the other?
2006-07-23 13:36:17 · 7 answers · asked by stupidbushtricks 2 in Science & Mathematics ➔ Mathematics
OK, no one seems to be getting this.
The chances of two random people BOTH being brothers-in-law AND sharing the same birthday HAVE to be higher than JUST the chances of two people sharing the same birthday.
If you meet a guy in a room, and find out he's got the same birthday as you, that's wild.
But if you find out that he's ALSO, say, married to your cousin, THAT's more remarkable, and I believe, a less likely occurrence.
SO I'm trying to figure out the odds of my wife's brother sharing my birthday AND of my being married to his sister.
2006-07-23 14:04:42 · update #1
Probability = P1 x P2
P1 = probability of two persons sharing the same birth date = 1/365 (keeping it simple and ignoring, 29 Feb and 365.25 days variants for now).
P2 = probability of a relationship between the two parties.
If we say that P2 = 1, it would be hard to believe (I think this is where you are coming from.)
So, what is P2?
It would have to be n/m, n is the number of those that will fulfill the brother-in-law requirement and m is the sample size of potential candidates that one will choose from for a marital partner. However, this may be a biased set, since sometimes, we do get to meet our relatives' friends and this depends on the cultural context as well, for example arranged marriages, etc.
2006-07-23 19:03:38 · answer #1 · answered by ideaquest 7 · 2⤊ 1⤋
There are 366 possibilities for birthdays. February 29 occurs once every four years, except for 2100, 2200, 2300, 2500, 2600, 2700, 2900......)
For two birthdays to be the same for any day of the year other than February 29:
(1/365)*(1/365)=1/1333,225
For two people to have the same birthday as February 29:
(1/(3*365+366) ) * (1/(3*365+366 ) ) =
1/1461*1/1461=1/2,134,521
2006-07-23 14:03:07 · answer #2 · answered by lager57 4 · 0⤊ 0⤋
Whether they marry sisters does not affect the odds of two people having been born on the same day of the year. It will happen once in every 365. The odds would only be worse than that if you picked a particular day of the year -- say July 4th.
2006-07-23 15:11:04 · answer #3 · answered by AardVark 2 · 0⤊ 0⤋
It does not matter who you meet, the odds remain the same. Your brother-in-law was probably not selected on the basis of his birthdate; therefore there is no connection and will not be a change in the statistics.
A non-random sampling implies that some selection criterion was applied. Suppose it was hair color. This would not be related to a birthdate. Therefore, the odds remain the same - 365:1.
2006-07-23 15:16:21 · answer #4 · answered by aichip_mark2 3 · 0⤊ 0⤋
John is right in that we assume that there is nothing about being brothers-in-law that will pre-select the birthdays.
Now, you could put conditions on it like, the sisters are twins. This may or may not preselect for them to marry twins, as well.
However, from a non-statistics and elementary probability point of view, I think it is safe to assume that there is no corelation between birthdays and marrying sisters.
2006-07-23 13:49:45 · answer #5 · answered by tbolling2 4 · 0⤊ 0⤋
I think it's still 1:365.25
It's still random, we don't choose our brothers-in-law.
If it seems wilder, it's only because you have far fewer brothers-in-law than strangers in a room.
2006-07-23 13:41:58 · answer #6 · answered by John C. 3 · 0⤊ 0⤋
The better question is . . . who cares?
2006-07-23 13:39:42 · answer #7 · answered by i_troll_therefore_i_am 4 · 0⤊ 0⤋