The math that explains why bell curves are everywhere

(quantamagazine.org)

56 points | by ibobev 2 days ago

6 comments

  • mikrl 2 hours ago
    Great article. Personally I have been learning more about the mathematics of beyond-CLT scenarios (fat tails, infinite variance etc)

    The great philosophical question is why CLT applies so universally. The article explains it well as a consequence of the averaging process.

    Alternatively, I’ve read that natural processes tend to exhibit Gaussian behaviour because there is a tendency towards equilibrium: forces, homeostasis, central potentials and so on and this equilibrium drives the measurable into the central region.

    For processes such as prices in financial markets, with complicated feedback loops and reflexivity (in the Soros sense) the probability mass tends to ends up in the non central region, where the CLT does not apply.

    • parpfish 2 hours ago
      As to ye philosophy of “why” the CLT gives you normals, my hunch is that it’s because there’s some connection between:

      a) the CLT requires samples drawn from a distribution with finite mean and variance

      and b) the Gaussian is the maximum entropy distribution for a particular mean and variance

      I’d be curious about what happens if you starting making assumptions about higher order moments in the distro

      • ramblingrain 52 minutes ago
        It is the not knowing, the unknown unknowns and known unknowns which result in the max entropy distribution's appearance. When we know more, it is not Gaussian. That is known.
      • derbOac 1 hour ago
        IIRC the third moment defines a maxent distribution under certain conditions and with a fourth moment it becomes undefined? It's been awhile though.

        If I'm remembering it correctly it's interesting to think about the ramifications of that for the moments.

      • sobellian 2 hours ago
        IIRC there's a video by 3b1b that talks about that, and it is important that gaussians are closed under convolution.
        • gowld 17 minutes ago
          That makes it an equilibrium point in function space, but the other half is why it's an a global attractor.
      • orangemaen 1 hour ago
        [dead]
    • benmaraschino 2 hours ago
      You (and others) may enjoy going down the rabbit hole of universality. Terence Tao has a nice survey article on this which might be a good place to start: https://direct.mit.edu/daed/article/141/3/23/27037/E-pluribu...
  • tsunamifury 0 minutes ago
    Bell curves are everywhere because all properties clump in some way.

    This is a tautology to the extreme.

  • bluGill 21 minutes ago
    100 year floods are not happening more often in most cases - it is just that the central limit therom teachs us the 10 year flood is almost as high water as the 100 or even 1000 year flood.
    • gowld 19 minutes ago
      Explain?

      What are "most cases"?

  • fritzo 2 hours ago
    Hot take: bell curves are everywhere exactly because the math is simple.

    The causal chain is: the math is simple -> teachers teach simple things -> students learn what they're taught -> we see the world in terms of concepts we've learned.

    The central limit theorem generalizes beyond simple math to hard math: Levy alpha stable distributions when variance is not finite, the Fisher-Tippett-Gnedenko theorem and Gumbel/Fréchet/Weibull distributions regarding extreme values. Those curves are also everwhere, but we don't see them because we weren't taught them because the math is tough.

    • BobbyTables2 1 hour ago
      It also took me a little while to realize “least squares” and MMSE approaches were not necessarily the “correct” way to do things but just “one thing we actually know how to do” because everything else is much harder.

      We can use Calculus to do so much but also so little…

    • gowld 12 minutes ago
      Most things aren't infinite or extreme, though. Almost by definition, most phenomena aren't extreme phenomena.
    • AndrewKemendo 2 hours ago
      That’s exactly the right take and the article proves it:

      Statisticians love averages so everywhere that could be sampled as a normal distribution will be presented as one

      The median is actually more descriptive and power law is equally as pervasive if not more

    • orangemaen 1 hour ago
      [dead]
  • gowld 1 hour ago
  • DroneBetter 2 hours ago
    I hate Quanta a lot

    a vast amount of fluff for less than a college statistics professor would (hopefully) be able to impart with a chalkboard in 10 minutes, when Quanta has the ability to prepare animated diagrams like 3Blue1Brown but chooses not to use it

    they could go down myriad paths, like how it provides that random walks on square lattices are asymptotically isotropic, or give any other simple easy-to-understand applications (like getting an asymptotic on the expected # of rolls of an n-sided die before the first reoccurring face) or explain what a normal distribution is, but they only want to tell a story to convey a feeling

    they are a blight upon this world for not using their opportunity to further public engagement in a meaningful way

    • KnuthIsGod 23 minutes ago
      3Blue1Brown

      Seems a bit like Ted Talks. Lightweight popcorn for the simple minded.

    • tptacek 2 hours ago
      A lot of times on HN when a math topic comes up that isn't about 3b1b, someone will jump in to say "this isn't as good as 3b1b". Last time I saw that, I was moved to comment:

      https://news.ycombinator.com/item?id=45800657

      3b1b doesn't have the same goal as Quanta, or as introductory guides. It's actually not that great a teaching tool (it's truly great at what it is for, which is (a) appreciation and motivation, and (b) allowing people to signal how smart they are on message board threads by talking about how much people would get out of watching 3b1b).

      This is prose writing about math. It's something you're meant to read for enjoyment. If you don't enjoy it, fine; I don't enjoy cowboy fiction. So I don't read it. I don't so much look for opportunities to yell at how much I hate "The Ballad of Easy Breezy".

      • bmenrigh 2 hours ago
        I don’t fault Quanta (or 3b1b) for being the way they are. Each is serving their goal audience pretty well.

        My compliant is only that there should be a dozen more just like them, each competing with each other for the best, most engaging math and science content. This would allow for more a broader audience skillevel to be reached.

        As it stands, we’re lucky even to have Quanta and 3b1b.

        I think there is hope though, quite a few new-ish creators on YouTube are following in Grant’s footsteps and producing very technically detailed and informative content at similar quality levels.

      • paulpauper 58 minutes ago
        there is no getting around that learning math requires actually having to buckle down and read and do math . A video will not suffice.
        • tptacek 57 minutes ago
          Couldn't agree more, which is why I think it's odd to suggest that a pop-sci magazine article is somehow a disservice that 3b1b would correct.