The Birthday Paradox Experiment
52 points by thunderbong 8 hours ago | 45 comments
  • chrismorgan 6 hours ago |
    (2018)
  • bsimpson 6 hours ago |
    It's interesting how uniform the birthday distribution at the end looks. I'd expect more seasonality (e.g. more babies conceived in the cold, dark months).
    • kibibu 5 hours ago |
      Weird how, coincidentally, 3x as many people are born on Nov 15 as any other day
      • boomboomsubban 5 hours ago |
        Nope, Jan 1st has about the same amount. So the lazy liars and those wondering what the dialogue says if they share a birthday with the prompt.
      • senectus1 4 hours ago |
        is that US Nov 15th?

        Cause I was born in Australia on Nov 14th :-D

        • sevg 4 hours ago |
          Wouldn't that be US Nov 13th?
    • unsnap_biceps 5 hours ago |
      I wonder if modern living has pretty much made this a non-factor. We have things to entertain ourselves regardless of the duration of light outside and so we're no longer left sitting at home in front of the fire bored, we're sitting at home in front of the fire playing with our phones. Less likely to bow-chicka-bow-wow out of boredom.
    • kqr 5 hours ago |
      Remember that the seasons are inverted on the other side of the equator. (Granted, the population splits something like 90/10 but still.)
      • ropejumper 4 hours ago |
        There are relatively few people living south enough to have "cold and dark winters," anyway. The northern hemisphere is much more concentrated towards the north.

        Most people live above 35°S where, at the most extreme, winter days are about 10 hours and a half long (plus about an hour of decent twilight). Temperatures obviously vary depending on region but they don't really get much below 10°C as far as I know.

        So really, it's more like mostly bright and somewhat chilly.

    • bambax 4 hours ago |
      Yes, it's also extremely unlikely.

      The actual distribution in developed countries is not uniform: there is a spike at the end of September (because many more people make babies at or around New Year's Eve) and a considerable drop on Dec. 25th (because people will avoid that date and provoque the birth some days before in case it might happen).

      Also, on the site there is a huge spike on Nov. 15 which, incidentally, is the birthday of the author: maybe they tested it many times?

      • profsummergig 3 hours ago |
        Why do people not like having their child be born on Xmas?
        • mrandish 3 hours ago |
          It's that some small percentage of births are intentionally induced for various medical reasons. Sometimes because the baby is full term but labor is not starting on its own yet. Other times an OB will attempt to rotate a full term baby that's in breach position to avoid needing a C-section surgery. Successful or not, this rotation procedure has a chance of triggering labor (which is why they wait until the baby is at or close to full term before trying it).

          Bottom line: hospitals are short staffed on Xmas so they set scheduled procedures which may induce labor for the day before or the day after whenever possible which preserves their limited capacity on Xmas for unscheduled births and emergencies.

        • KineticLensman 2 hours ago |
          My sister’s birthday is a few days after Christmas. She hated this as a child because once her birthday was over there was almost a whole year without any prezzies
      • Miraltar 3 hours ago |
        15th november are valentine's babies, I'm not surprised about this spike
        • boomboomsubban 3 hours ago |
          Any such spike would be over weeks, not heavily concentrated on one day. It's clearly people taking the quiz twice to see what it says if you match the creators birthday.
  • phito 5 hours ago |
    I really like the fact that it's using the previous user birthdays. Unfortunately today is my birthday and I think a lot of people are entering today's date as their birthday, I got 6 matches... :)
    • notduncansmith 5 hours ago |
      Happy birthday :)
    • zeristor 4 hours ago |
      Nothing unfortunate about having a birthday today.

      Happy Birthday!

    • block_dagger 4 hours ago |
      Happy birthday to you and me, fellow Scorpio.
    • jll29 3 hours ago |
      Plenty of November birthdays because it's cold in Februrary (11 - 9 = 2).
    • rukuu001 2 hours ago |
      I have FOUR significant people in my life with November 15th birthday, so I just groaned when I saw the author’s birthday.

      And happy birthday phito :)

    • phito an hour ago |
      Thanks for all your birthday wishes <3
  • zero0529 5 hours ago |
    Incredible site that also is very mobile friendly!
  • nuancebydefault 4 hours ago |
    I'm a bit confused. Maybe i did not pay enough attention but... Does it stop the first simulation when there are _any_ 2 people with a same birthday, or when someone has _my_ birthday? I think the latter will take more people on average.
    • dvt 4 hours ago |
      The website is being purposefully obtuse and the birthday paradox is a "paradox" because it's sometimes formulated in the same way as the website. In other words, you are completely correct that the intersection of any two birthdays is significantly more likely than the intersection of a particular birthday.
    • unklefolk 4 hours ago |
      The Birthday Paradox is about a group of people sharing at least one birthday between them. If it was at least 1 person out of 23 people sharing YOUR birthday then the odds would be 1 - (364/365)^23 which is around 0.06 (or 6% chance). So, yes, this scenario is a lot less likely.
      • nuancebydefault 4 hours ago |
        Thanks for that. I believe the fact that it asks for a birthday puts people's brain in a mode where the awe at the end is bigger.

        Related, having flat tires on a car seems to come in little bursts,like 10 years none and then 2 in a year.

        • Cthulhu_ 3 hours ago |
          Can confirm, but both times it was a screw or something in my tire so I presume someone dropped some at a point of my commute.
        • jyounker 2 hours ago |
          I'd expect flats on cars to be correlated. People tend to buy tires in sets, so age related factors affect them all. Similarly they tend to get driven on the same roads, so are subjected to similar environmental damage.

          Also, people are good at noticing patterns that don't exist, so that's a possibility too.

  • zeristor 4 hours ago |
    I was going to try it again with February 29th
  • revskill 4 hours ago |
    Scripted.
  • tosser0001 4 hours ago |
    An analog of the birthday paradox that gets me all the time is what I think of as The Locker Room Paradox. This is where when I go into the locker room after working out and the guy who comes in behind me ends in the locker right next to mine. So there’s two of us in a big empty room awkwardly jostling away.
    • DiscourseFan 3 hours ago |
      >awkwardly jostling away

      I imagine it’d be more fun in a group setting?

    • quietbritishjim 3 hours ago |
      (Apologies for the fun-ruining comment.)

      For it to be a true analogue if the birthday paradox, it would have to happen rarely to you individually, but surprisingly often to one pair of people in the locker room when there are a smallish number in there.

    • xydone 2 hours ago |
      Assuming you don't have an automated system to give away locker keys, wouldn't this be explained by the fact that gym front desk is more likely to give out the lowest number available and as you took X, they will give out X+1 for the next person?
      • dustincoates 2 hours ago |
        I've never been to a gym where you're assigned a locker for the day (or given a key). Either you have one permanently assigned (rare) or you go in and find one that isn't occupied.
        • xydone 2 hours ago |
          Ohh, I see! At my gym, locker keys are given to you by the front desk and you put in something as deposit (such as your gym card or whatever you wish) and on your way out you give the key and you get your deposit back.
  • tmountain 3 hours ago |
    I had a hard time believing the birthday paradox when I first heard about it years ago, so I modeled it with Clojure/Incanter and the results were spot on. Really interesting and fun paradox.
  • k__ 3 hours ago |
    Aren't there birthdays that are more likely than others?
    • tromp 2 hours ago |
      There is one birthday that is far less likely than others, courtesy of leap years. All other are roughly similar, with small seasonal variation.
      • nicoco 2 hours ago |
        And no moon cycle related variations, despite a popular (false) belief. It always amazes me that the superstition is spread even among (some, of course not all) midwifes!
  • badcppdev 2 hours ago |
    I was going to say that I was disappointed that the page didn't show the formula for the probability and show how it changes as you change the number of people in the room.

    Then I checked the wiki page and understood that some of the maths is actually quite fiendish (for non mathematicians anyway) - https://en.wikipedia.org/wiki/Birthday_problem

  • lukol an hour ago |
    While the math is clear, I'm a bit annoyed by the label "paradox" as the whole setup is too simplistic and reductionistic.

    The actual chance of being in the same room with someone who shares your birthday needs to include other factors like your socioeconomic background, the cultural environment you are in, your present location, and certain historical facts.

    Without having done the math, I'm fairly certain that a member of the baby boomer generation in New York has a higher chance of meeting their birthday sibling than a 12-year-old in a rural part of Australia.

    • wodenokoto an hour ago |
      Wait, what?

      Are you saying that income influences what time of the year you are born?

      The paradox doesn’t talk about meeting your birthday sibling, but meeting two people who are birthday siblings.

      So by the time the 12 year old has met ~20 people, there is a 50/50 chance that amongst those 20, there’s a birthday pair.

    • tomtom1337 an hour ago |
      Could you explain more what you mean? Other than certain holidays perhaps causing a higher chance of alone time for future mom and dad, I don’t see what you mean, and I don’t understand your last paragraph.