 When is your birthday?  


Author  Message 

Bub
Registration date : 20071215
 Subject: When is your birthday? Mon Apr 04, 2011 11:17 pm  
 This video says that the odds are about 50:50 that 1 in 23 people will have the same birthday. http://www.wimp.com/justcoincidence/ Mine is August 22 8/22
Last edited by Bub on Tue Apr 05, 2011 9:33 pm; edited 1 time in total 

 
Doc Manhattan
Age : 40 Location : Land of Steady Habits Registration date : 20080526
 Subject: Re: When is your birthday? Tue Apr 05, 2011 12:03 am  
 Love it!
(I'm a math nerd at heart... even if my grades were never very good in the subject.) 

 
Bub
Registration date : 20071215
 Subject: Re: When is your birthday? Tue Apr 05, 2011 8:59 pm  
 Does anyone want to see if these numbers are correct by listing your birthday?
Last edited by Bub on Tue Apr 05, 2011 9:34 pm; edited 1 time in total 

 
CLRV
Age : 34 Location : Tulsa, Oklahoma Registration date : 20101229
 Subject: Re: When is your birthday? Tue Apr 05, 2011 9:04 pm  
 

 
SmokeyTweed
Age : 35 Location : Edmonton, Alberta Registration date : 20081013
 Subject: Re: When is your birthday? Tue Apr 05, 2011 9:17 pm  
 

 
Cody V
Age : 32 Location : Memphis, TN Registration date : 20100912
 Subject: Re: When is your birthday? Tue Apr 05, 2011 9:26 pm  
 

 
Bub
Registration date : 20071215
 Subject: Re: When is your birthday? Tue Apr 05, 2011 9:32 pm  
 Thanks, we just need the month and day 

 
Frost
Age : 40 Location : Somewhere near Philly, PA Registration date : 20091231
 Subject: Re: When is your birthday? Tue Apr 05, 2011 9:50 pm  
 

 
mark Admin
Registration date : 20080702
 Subject: Re: When is your birthday? Tue Apr 05, 2011 10:33 pm  
 7/8 _________________ .
Sometimes I wake up grumpy. Other times I let her sleep. .


 
Milan
Registration date : 20110317
 Subject: Re: When is your birthday? Tue Apr 05, 2011 10:53 pm  
 

 
Doc Manhattan
Age : 40 Location : Land of Steady Habits Registration date : 20080526
 Subject: Re: When is your birthday? Tue Apr 05, 2011 11:03 pm  
  Bub wrote:
 Does anyone want to see if these numbers are correct by listing your birthday?
As the video says, there's no meaning in testing an assertion about absolute probability; it's just true. (Not to be confused with sports book odds, which are really just educated guesses.) We have more than 366 members here, so the probability of duplicate birthdays on the board is 100%. If we had 1 member, the chance would be 0%. The odds can be figured out absolutely for every number in between those limits. The odds of 2 members out of any 22 picked at random sharing a birthday is ~50% That means that the odds are just as good that we wouldn't see a match, but it doesn't change the odds. Whether we saw a match or not, the 5050 odds would be demonstrated equally. When you toss a coin and it comes up heads, that doesn't mean that the tails side stops existing or coming up on half of any other tosses. 

 
CLRV
Age : 34 Location : Tulsa, Oklahoma Registration date : 20101229
 Subject: Re: When is your birthday? Tue Apr 05, 2011 11:13 pm  
 My brain hurts now Doc. 

 
Doc Manhattan
Age : 40 Location : Land of Steady Habits Registration date : 20080526
 Subject: Re: When is your birthday? Wed Apr 06, 2011 9:05 am  
  CLRV wrote:
 My brain hurts now Doc.
I should just stick to teaching English! 

 
Ataki
Age : 39 Location : San Antonio, TX Registration date : 20101124
 Subject: Re: When is your birthday? Wed Apr 06, 2011 9:36 am  
 

 
Bub
Registration date : 20071215
 Subject: Re: When is your birthday? Wed Apr 06, 2011 10:05 am  
 Doc said  Quote :
 We have more than 366 members here, so the probability of duplicate birthdays on the board is 100%. If we had 1 member, the chance would be 0%. The odds can be figured out absolutely for every number in between those limits.
I am just curious how long it takes to find two people who have the same birthday. Initially I thought that it would be a much larger number. Doc, if you teach 5 classes with 30 people how many classes would have 2 people with the same birthday? 

 
LIPIPE
Age : 70 Location : Setauket,Long Island and upstate Granville, New York Registration date : 20101218
 Subject: Re: When is your birthday? Wed Apr 06, 2011 10:47 am  
 Bronx Bill and I (Bronx Marty) have the same birthday born one year apart in the same NYC borough, home of the New York Yankees...May 13th 

 
Bub
Registration date : 20071215
 Subject: Re: When is your birthday? Wed Apr 06, 2011 12:32 pm  
 so far 9 birthdays and no matches 

 
Doc Manhattan
Age : 40 Location : Land of Steady Habits Registration date : 20080526
 Subject: Re: When is your birthday? Wed Apr 06, 2011 1:38 pm  
  Bub wrote:
 Doc, if you teach 5 classes with 30 people how many classes would have 2 people with the same birthday?
Probability says the odds of a duplicate in each class are independent. With 30 people, the odds are good (about 57%) I'll have duplicate birthday in that class, but whether I do or don't doesn't affect those same odds in either class, the same way a die rolling 6 doesn't affect a separate die from rolling any number. But I can extrapolate the odds of any possible combination of duplicates by multiplying the independent odds, which say the odds are on 3 matches, 2 nomatches, but they're very narrow odds... I wouldn't bet $5 on it, to put it that way. 

 
RealtorFrank
Age : 34 Location : Nashville, TN Registration date : 20101121
 Subject: Re: When is your birthday? Wed Apr 06, 2011 2:28 pm  
 

 
Bub
Registration date : 20071215
 Subject: Re: When is your birthday? Wed Apr 06, 2011 3:46 pm  
 Doc, I agree, but can you ask them? They might find the results surprising. I still find it hard to believe that in a classroom the odds are 50:50 to find 2 people with the same birthday. 

 
Doc Manhattan
Age : 40 Location : Land of Steady Habits Registration date : 20080526
 Subject: Re: When is your birthday? Wed Apr 06, 2011 10:18 pm  
 Bub, I'll show you how it works:
With two people in the class (Mr. A and Ms. B), the odds are a little better than 1:366. Not great, but not astronomical.
Dr. C joins the class. Now the odds of a match are 3:366 = 1:122, because the total chances for a match are AB, AC, BC. Getting close to 1% odds already!
Add Mr. D, it becomes 5:366 = 1.3% odds for a match in AB, AC, AD, BC, BD, CD.
The odds are always the total combinations/366, which = the sum of the numbers from 2 to (p1) / 366, where p = number of people in class.
At 23 people, that's 252/366 = easily more than 50%. 

 
Frost
Age : 40 Location : Somewhere near Philly, PA Registration date : 20091231
 Subject: Re: When is your birthday? Wed Apr 06, 2011 10:53 pm  
 Odds are a mathematical construct...life is not. You can go to 10,000 different classrooms with 100 students in each one and never find two people with the same birthday. Doesn't change the fact the odds were 50/50 or whatever every time. If the weatherman says there is a 99% chance of rain today, but it was 70 degrees and not a cloud in the sky all day; he wasn't wrong...it's just that today decided to be in that 1%. 

 
Doc Manhattan
Age : 40 Location : Land of Steady Habits Registration date : 20080526
 Subject: Re: When is your birthday? Thu Apr 07, 2011 9:35 am  
  Frost wrote:
 ou can go to 10,000 different classrooms with 100 students in each one and never find two people with the same birthday. Doesn't change the fact the odds were 50/50 or whatever every time.
'Zactly! An unfortunately, hard probability statistics don't make people act rationally (especially when the chance of money or convenience is dangled in front of their faces.) 

 
Rob_In_MO
Age : 45 Location : Park Hills, MO Registration date : 20110119
 Subject: Re: When is your birthday? Thu Apr 07, 2011 9:48 am  
 

 
MartinH
Age : 48 Location : The South Registration date : 20110104
 Subject: Re: When is your birthday? Thu Apr 07, 2011 7:04 pm  
 9/6 so not giving the year. lol 

 
 When is your birthday?  
