[QUANTITY] Why quantities always have errors (Was: Re: [QUANTIT] Use-cases, role in larger scheme)

[Brian Thomas]

On Monday 17 November 2003 02:45 pm, Thomas McGlynn wrote:
> Hmmm...  It's probably senseless to get involved in this, but it seems to
> me that there is a confusion here between quantity and measurement.  

	When was "measurement" defined? How can there be confusion then?
	I believe that it was pointed out some time back that the quantity that I 
	(and some others) speak of overlaps with some people's definition of 
	measurement. There are no exact words in the DM lexicon, yet.

> An Error
> is a property of a measurement.  Depending upon how the measurement is done
> it may be appropriate to think about an error as something that can be
> modeled a variation around a measurement. 

   Accepting your definitions for the moment, yes, this is true. 

>  Quantities do not need to be
> measured, they can be defined, calculated, simulated ....  And even if they
> are measured, they may not be susceptible to an error value.

   Hmm. how useful is it to have separate classes for these types of quantities?

   I believe that quantities are framed by their *usage*, not their *origin*. IF we need to have 
   a universal atom that we can pass around, clip apart/rebuild, and search for, 
   then we don't care if something is a measurement, or a simulation or not. We
   need to lump all of the types of error together in the interface in order to be able
   to compare, clip, paste, parts which are measured, defined, calculated values.

   Having separate classes for measurement, calculation, simulation quantities
   doesn't get you much (or anything).

>
> Regardless, I'm still much more interested in how this or any data model
> gets me closer to being able to write software tools for astronomers.  I'm
> not sure how this discussion really helps.

   As am I. This is why I frame the quantity class needs in terms of its usage, rather
   than its scientific origin. Framed in terms of its usage (requirements) the quantity
   forms a usefull item for the purposes of search, data fusion and exchange.

   Cheers,

	=b.t.

>
> 	Regards,
> 	Tom McGlynn

-- 

  * Dr. Brian Thomas 

  * Code 630.1 
  * Goddard Space Flight Center NASA

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