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Ben Kovitz explained:
>
> In science & engineering, to say, "You aren't even in the right order
> of magnitude" means, "You are so far off, making improvements to the
> details won't make any real difference. You have to change your whole
> approach." Or, "You dope, you got the decimal point in the wrong
> place."
>
> To say, "We've improved by an order of magnitude" means, "Relative to
> this improvement, most incremental improvements are in the realm of
> insignificant digits or experimental error."
>
To me, "order of magnitude" fits in with the concept of the "envelope-back
calculation." Ben expressed this idea nicely.
Engineers should be familiar with the term "envelope-back calculation."
Basically, it's a quick feasibility check of the numbers behind a particular
system, process or design. The idea is to rule out a given thing or provide
a numerical estimate based on ballpark calculations that can fit on the back
of an envelope. It's a good use of those estimation rules and mathematical
shortcuts you might have learned in school. For example, at the grocery
checkout: "The eggs are a little over a dollar, the milk is a little under
two dollars, the chips are a little under a dollar ... so the bill should be
about four dollars. I only have three-fifty, so I need to look at this more
closely, or maybe I should just put back the chips."
Systems calculations can get hairy, but often an engineer can rule out
something by doing such a ballpark calculation. For example, a biochemical
engineer might be selecting a reactor for a particular process. After
looking at a proposed reactor design, he might ballpark the operational
parameters and, considering the process he has in mind, he could do an
envelope-back calculation. Let's say his numbers show that the dissolved
oxygen level imparted by the reactor is an order or magnitude lower than he
requires for his process. With a few minutes of effort, he knows it's a big
no-go and it's time to move on to the next design.
Yes, this is a meatball process, but it fits in with Ben's description:
"You are so far off, making improvements to the details won't make any real
difference. You have to change your whole approach." In this case, the
engineer only had to spend a few minutes ruling out a reactor design,
because the numbers are so far off of what he needs that they're apparent
with a loosey-goosey calculation.
It's a good term for technical writers to know, if for no other reason than
to understand a bit more about their SMEs. As for the practice of
envelope-back calculation, I do it all the time, pretty much automatically.
"If I compress the jpg by this much, then the file should be this size or
less." "If the original graphic is this big, then I'll need to size it to
this much to make it look okay at 100% in PDF." "If the plain text has this
many pages, it should end up as that many pages in my Word template.
Thanks to online bill payment, I have a lot of old envelopes if anyone wants
some.
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