Awesome Birthday Paradox

Discussion in 'Community Discussion' started by Simgar988, Mar 15, 2010.

  1. Simgar988 macrumors 65816

    Simgar988

    Joined:
    Jul 22, 2009
    Location:
    UYBAATC
    #1
    365/2= 182?

    90?

    70?

    Actually, the answer is 23 people. For a 99% chance the answer is 57 people.

    VERY INTERESTING AND COUNTER-INTUITIVE!
    http://en.m.wikipedia.org/wiki/Birthday_problem?wasRedirected=true


    lets test! no cheating please!

    September 10

    For simplicity, disregard variations in the distribution, such as leap years, twins, seasonal or weekday variations, and assume that the 365 possible birthdays are equally likely. Real-life birthday distributions are not uniform since not all dates are equally likely
     
  2. sycho macrumors 6502a

    sycho

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  3. Simgar988 thread starter macrumors 65816

    Simgar988

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    UYBAATC
  4. ucfgrad93 macrumors P6

    ucfgrad93

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  5. OzExige macrumors 6502

    OzExige

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  6. thejadedmonkey macrumors 604

    thejadedmonkey

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    #6
    My 2 best friends and my girlfriend all have the same birthday as me... go figure.
     
  7. flopticalcube macrumors G4

    flopticalcube

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    In the velcro closure of America's Hat
    #7
    That must make for one heckuva birthday party.

    My eldest son has my late mother's birthday and my middle child has my brother's. My wife shares hers with Hitler... I'm not going to add anything to that. :p
     
  8. diamond3 macrumors 6502a

    Joined:
    Oct 6, 2005
    #8
    I'm more interested in how you do the stats for that problem. I should remember, but i had a terrible teacher and it has been a few years.

    2/26
     
  9. jhero macrumors 6502

    jhero

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    #9
    Agreed. The stats would be very interesting to see.

    10/21
     
  10. bgd macrumors regular

    Joined:
    Aug 30, 2005
    Location:
    SG
    #10
    And why is this? Time of year, more kids conceived in winter for example?

    Remember a story about a powercut in Britain leading to a spike in births nine months down the track.

    So what sort of things make some dates less likely than others?
     
  11. Counterfit macrumors G3

    Counterfit

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    sitting on your shoulder
    #12
    My paternal grandmother was born exactly one year after (before maybe? I forget who's older) one of her sisters, on Valentine's Day. Pretty neat.

    Then, she not only had one kid on her birthday, but two, also exactly one year apart. And just this past June, my sister-in-law gave birth on her birthday. So in my family, it's pretty damn likely.
     
  12. fireshot91 macrumors 601

    fireshot91

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    Location:
    Northern VA
    #13
    Happy Birthday?


    3/22
     
  13. swiftaw macrumors 603

    swiftaw

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    Jan 31, 2005
    Location:
    Omaha, NE, USA
    #15
    Ok, if you have n people then there are 365^n different ways they could have birthdays.

    Of those ways, 365! / (365-n)! of them are such that everyone has a unique birthday.

    Now, under some simplifying assumptions (no leap years, uniform distribution of births over all days) the probability that a group of n people all have unique birthdays is [365! / (365-n)!] / [365^n]. Therefore, the probability that at least two people share a birthday is

    1 - [365! / (365-n)!] / [365^n]

    If you plug in n=23, the answer is around 50%, the bigger n is, the higher the probability (obviously).
     
  14. comictimes macrumors 6502a

    comictimes

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    #16
    Hey mine too- March 22 :)
     
  15. yg17 macrumors G5

    yg17

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    St. Louis, MO
    #17
    Hmm, I thought I was bad, I share my birthday with that bigoted bitch Anita Bryant. :p
     
  16. Dagless macrumors Core

    Dagless

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    Fighting to stay in the EU
    #19
    I shared a birthday with a friend at college. We were really similar too; had the same humour, same hobbies, same weird twisted mind. Jan 8th.
    I think I share a birthday with my great grandmother too. Who also died on the day my brother was born.
     
  17. pit29 macrumors 6502a

    pit29

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    The Golden State
    #20
    Same here! But I guess now this thread is biased...
     
  18. Ttownbeast macrumors 65816

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    May 10, 2009
    #21
    One of the biggest conception times coincides with the start of the new school year here around the end of August, or beginning of September resulting in lots of May/June babies.
     
  19. Doctor Q Administrator

    Doctor Q

    Staff Member

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    Location:
    Los Angeles
    #22
    By my count it took us only 10 people, not 23, to get a birthday match (March 22, between fireshot91 and pit29). The odds were good that it wouldn't take more than a forum page, and we beat even those odds!

    Please don't post more birthdays in this thread. You are welcome to talk about this or other surprising facts of probability and math.
     
  20. renewed macrumors 68040

    renewed

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    Mar 24, 2009
    Location:
    Bemalte Blumen duften nicht.
    #23
    Just a fun fact I'm sure many of you know, but it's interesting nonetheless. If you go to the main Mac Forums page and scroll all the way down, you can see all the birthdays for that day. It's crazy how many members have the same birthday on any given day.
     
  21. Roric macrumors regular

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    Sep 29, 2005
    Location:
    WI
    #24
    At lunch today someone mentioned that they knew someone who was born on his dad's 30th birthday... and his own child is due to be born about 6 days after his 30th. Just think if the baby decides to come 6 days early. Three generations exactly 30 years apart from each other.
     
  22. fireshot91 macrumors 601

    fireshot91

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    Jul 31, 2008
    Location:
    Northern VA
    #25
    I didn't mention this before (For the sake of another person getting another birthday as me), but my mom's birthday is also the same day.
     

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