From: | Gurth gurth@******.nl |
---|---|
Subject: | [OT] A rant about rounding off that I need to get off my chest... |
Date: | Mon, 18 Oct 1999 13:09:58 +0200 |
was...
> The pound to kg conversion is more accurate if you
> multiply by 0.4536, rather than dividing by 2.2.
<rant>
Hasn't anybody in the imperial-to-metric-conversion world heard of
significant digits (or whatever the concept is called in English)? There's
no point in multiplying by a factor of 0.4536 if you only know the mass in
whole pounds -- your overall accuracy will only be as good as the part of
the conversion that has the lowest accuracy. One digit more is usually
handy, so you can round off afterward, but more than that is unnecessary.
In other words, saying that 10 lbs (2 significant digits) equals 4.536 kg
(4 significant digits) is nonsense; 4.5 kg (2 significant digits) is as
accurate as you can get. Now if you'd measured the 10 lbs to be _exactly_
10.000000 lbs (8 significant digits), then you could say it equals 4.536
kg (4 significant digits), but you can't say it's 4.5360000 (8 significant
digits) because the lowest accuracy is now in your multiplication factor.
Thus, in most cases, dividing by 2.20 is accurate enough.
For those who think the above is too complicated, do it like this: round
off to whatever is normal in what you're measuring in. If you measure body
weight, in countries using imperial measurements you would round off to
whole pounds. So, when converting to kilograms, you round to the nearest
kilo. Nobody says "I weigh 74.285 kg" -- rather, they'll tell you they
weigh 74 kg.
</rant>
--
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