[QUANTITY] Why quantities always have errors

[Brian Thomas]

On Monday 17 November 2003 02:30 pm, Patrick Dowler wrote:
> >       I for one believe it is "an error" to have a number without an
> > error.
>
> So presumably it is wrong to write the eqution for a parabola as y = x^2
> and not spec ify the error in "2"?? Last time I checked, 2 was a number.

	Yes, and it *does* have an error: "exactValueNoError". Often
	this is simply understood to be the case when you talk about a "defined"
	values. But how does the user know that your quantity was defined? You
	know it because you set the thing up. In the VO, we need to be explicit as
	having a mathematical equation isnt enough knowledge. You could also very 
	well have derived the relation ship from  least-squares fit of a parabola, then, 
	in that case, "2" had better have an error, and if it doesnt, then it *doesnt* 
	mean "exactValueNoError", instead it means "NoErrorBecauseImTooLazyToTellYou".

	Regards,

	-b.t.

-- 

  * Dr. Brian Thomas 

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  * Goddard Space Flight Center NASA

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