Accuracy is a sharpshooter's ability to hit close to the bullseye of a target. Precision is a sharpshooter's ability to produce a small "cluster" on a target, even if the cluster is not at the bullseye.
In the example I gave above, you don't care about the accuracy of the 69kv figure, but you *do* care that the two values are quite precisely the same!
Accuracy is how close a meter measuring a voltage of 120.000 volts is to displaying "120.000". Precision is reproducability, or ability to distinguish different values from each other. Consider a very high quality voltmeter with a zero adjustment that's misset. Perhaps it measures a voltage of 120.0000 as 117.2312, and it can do so repeatedly, and it measures a voltage of 121.0000 as 118.2312. Very precise, but not very accurate.
They can be labels, measurements or calculation. If I see a panel labeled "208V" that's a label. If I measure 208V in it with a voltmeter, that's a measurement. If I multiply 120V by sqrt(3) and get 208V, that's a calculation.
No, by itself it's just a number. A number has no history. It's likely to be such a calculation, it's not a measurement since no instrument is likely to be that accurate, and it's unlikely anyone (other than possibly yourself) would use it as a label.
As someone else just said, calculators and computers are stupid beasts. If a calculator is designed to use 10 digits, it will use 10 digits, regardless of whether all 10 of those digits are actually useful. My calculator can be told how many "sig figs" to display, and it does its calculations to its full precision internally, but it will only display the requested number of digits. If my calculator come up with "119.999999997" internally but is set for 4 sig figs, it'll display "120.0", as it knows how to round.
I can't think of *any* context where "69282.032302755 +/- 100" makes
*any* sense, even in a nonengineering context.
If any of the figures used are known to only two digits, that's the correct thing to do.
Precision in the arithmetic is more than just getting all the digits
This makes no sense.
It appears from some of the above, you're one of them.
I was talking about an engineering environment, where you do have to do so.
That would be wrong. If there is a need to identify further it should be written out. I wouldn't write "208" in isolation, although I might write "208 volts line to line between phase A and phase B in panel X" or some such.
Huh? If meaning is unimportant? Would you add random numbers in the middle of a document?
Not really. The numbers may be slightly different but still in range, if done correctly.
There you go. You wrote 600 instead of something silly like
601.4379256973140.
Good engineering would never specify a "1000 pound rope" for a load that may be 1005 pounds, but that's another story. You yourself would specify a 600V capacitor in the previous paragraph.
My point was that the circuitry won't be either precise or accurate enough to warrant that many digits.
If you're talking mathematics such as the volume of a sphere with radius of 1, this doesn't apply. To a mathemetician, 1 means 1 to an infinite number of digits. In fact, a mathematician wouldn't use decimals in that case, he'd write "4/3 {pi}" which is exact. ({pi} is the standard pi symbol, of course)
Thus no need for all those digits.
Yes, if you have 20 digits of significance, and you need all 20, don't use a value of sqrt(3) calculated to 5 digits. If you have only two sig figs, 5 digits is plenty.
You need to design the application better and/or get more accurate figures.
Chaos theory can show foolishness in what you do. If a butterfly flaps its wings at 0 meters means a hurricane hits New Orleans, but at 1 meter means it hits Miami, and you use a figure of 0.76327438925793845298 meters and you come up with an answer like "the west side of 5th St. 221.3213 millimeters from the curb, Clearwater FL", but we don't know where the butterfly is within a kilometer, we have no idea where the hurricane will be (if there even is one), and you'll do nothing but panic the residents of Clearwater.
They need to take Engineering 101 again and learn "sig figs".
No, this is a calculation.
What is the accuracy of the 230V figure? Assuming you mean the European standard voltage, which I believe mentions something like 6%, two sig figs is enough. 230 times sqrt(3) = 398.371blablabla, rounded to 2 sig figs gives 400V.
Yes, 400V. But not for the reason you say.