[QUANTITY] Why quantities always have errors

[Brian Thomas]

On Tuesday 18 November 2003 10:52 am, you wrote:
> My maths is out of date and I can't remember how a least squares
> works...! But as far as I recall, what you get is an approximation
> formulae to fit a bunch of points?  So then yes, you have a formulae
> with an error (but where that error goes and how it is defined for
> formulae is another issue).  But you don't when the formulae is derived
> or defined.
>
> But even so, it strikes me that you have situations where errors are
> relevent *and so should be included* and some where they are not *and so
> should not exist*.

	Yes, exactly.

>
> I think you are making the point that where there is no error with a
> value, we should make it clear whether it is because we don't know the
> error, or that there shouldn't be one.  I believe this discrimination
> sould be in given in the 'type', as (1) it shows much more readily when
> an error is relevent or not, and (2) it 'forces' the user of the type to
> consider the error, rather than habitually setting it to 'None'.

	Yes, agree, this is my point of view, and why I want "error" (or 
	actually, I'd call it "accuracy") on all quantities.

>
> A question: Can a simulation produce a value without an error?  I'm
> trying to remember the four formulae for building a star that should
> give you a flux density, and as far as I remember we got fixed values
> out, but it was a long time ago.

	Yep, you sure can. The Kurucz (sp?) models I remember using as
	an undergrad created 'exact' models of the photosphere/lines. 
	But you can also "simulate" the errors that you might get too.  I remember
	running the K-models thru a "resolver" that made the outputs of
	these models "look like" they had been observed at a certain resolution
	(e.g. I added noise, systematics, etc to the values of the model).

	Regards,

		=b.t.



>
> Cheers
>
> MC

-- 

  * Dr. Brian Thomas 

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  * Goddard Space Flight Center NASA

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