from Hacker News

The Back of the Envelope (1984) [pdf]

by tjalfi on 6/19/24, 5:08 PM with 8 comments

  • by Someone on 6/21/24, 12:02 PM

    Related is is “hemibel thinking” in operations research. https://apps.dtic.mil/sti/tr/pdf/ADA100421.pdf:

    “Having obtained the constants of the operations under study… we compare the value of the constants obtained in actual operations with the optimum theoretical value, if this can be computed. If the actual value is within a hemibel (…a factor of 3) of the theoretical value, then it is extremely unlikely that any improvement in the details of the operation will result in significant improvement. [When] there is a wide gap between the actual and theoretical results… a hint as to the possible means of improvement can usually be obtained by a crude sorting of the operational data to see whether changes in personnel, equipment, or tactics produce a significant change in the constants.”

    The term was invented while operations research was being invented: in World War Two. They also thought (rightfully so) that, once you’re within a hemibel for one problem, there are plenty of other problems where it is easier to get return on investment than when continuing on the problem at hand.

  • by hermitcrab on 6/21/24, 10:24 AM

    I read Bentely's 'Programming Pearls' many years ago. It was a real eye opener for a junior programmer. Thoroughly recommended.
  • by kragen on 6/21/24, 9:39 AM

    bentley is always worth reading

    it's interesting to reflect that the 'supercomputer' he cites is a hundred megaflops, about the same as a pentium pro in theory (though the super didn't have a cache to slow it down, so it was typically faster by over an order of magnitude.) i was using a pentium pro at the office when i first read this article. now my cellphone can manage tens of gigaflops if not hundreds

    the bob martin mentioned is http://www.ewaygroup.com/Bob-bio.html, not the better-known ignorant blowhard

  • by aaron695 on 6/21/24, 10:17 AM

    Learning how to do back of the envelope calculations is not simple, it needs to be taught and you need to work on problems [1]

    Everyone at university not doing an Imposter degree should have had it taught. It's not university level, but it's above average high school level I'd guess.

    I'm not sure how you would learn it online?

    [1] A classic problem is have you breathed in part of Hitlers last fart? It's not a hard calculation, but is gives you insights to CNN reporting microplastics being found in penises (Yes the Doomers have moved on past the testicles)