John F. Sowa wrote:
> EB> In particular, the VIM explicitly says that the magnitude of a
> > quantity is not knowable; it is only measurable, and every measurement
> > involves uncertainty. This is absolutely true, and it is the gospel
> > of the science.
>
> Not just the science of measurement, but *every* empirical science.
> This gets into serious questions of how granularity and uncertainty
> should be accommodated in an ontology.
>
Yes. The problem is that the scientific usage and the commercial usage
are significantly different in this area. (01)
> EB> But it means that you cannot say that a stretch of roadway is 1km
> > in length; you must say that it is 1km plus-or-minus 5 cm, if that
> > is the accuracy of your survey. And similarly, the label on your
> > Coke bottle that says "1 litre" and the label on your bag of
> > potatoes that says "10 lbs" is, according to the VIM, meaningless
> > without additional information.
>
> Such details are extremely dependent on the application -- they
> belong in the microtheories. For example, if somebody's height
> is listed as 180 cm, the granularity would not be relevant in a
> medical record. But if an airplane part has a length of 180 cm,
> a subcontractor that manufactures it must know the tolerance.
>
Being careful, 'tolerance' and 'uncertainty' are entirely different
things, as I mentioned in a previous email. Tolerance is the allowable
variance from the specified quantity, where that difference allows the
measured object to meet the functional and physical requirements;
uncertainty is the amount by which the measurement may be different from
the actual magnitude of the quantity. In complying with a tolerance,
one has to ensure that the worst case measurement error gives a value
that is still within the tolerance. (02)
Tolerance is certainly application-dependent. I don't think that
'uncertainty' is application-dependent (per se). Uncertainty is
dependent on the 'measurement', i.e., the measuring activity, and many
aspects thereof: -- procedure, equipment, environment, etc. I would
prefer that the units of measure ontology not go deeply into
'measurement' at all. My point was that 'measurement' is the meat of
the VIM, and that affects its way of modeling 'quantity'. (03)
> EB> Are we making a formal ontology for measurement science,
> > or a practical ontology for units of measure?
>
> Different kinds of "practical problems" will require different ways
> of treating (or ignoring) the tolerance. It's probably impossible
> to accommodate all the ways in a single, fixed formal theory.
>
Agree. The question, however, is to what degree we need to deal with
_uncertainy_. That, for example, may determine how we deal with the
concept 'quantity magnitude' -- the "unknowable" abstraction of the
particular quantities that underlies the definition of 'measurement
unit'. (Or maybe it doesn't, and that is the question.) (04)
The SI system is based on well-defined units, whose definitions are
specific measurements of particular physical quanitities (with known
uncertainties, I assume). And the VIM idea is that other measurements
are compared to those measurements in "traceable" ways that accumulate
the uncertainties involved. And the idea is that the accumulated
uncertainty is well within the tolerance of the application for which
the measurement is being made. (05)
If the nurse measures the patient's height at 180 cm, the nearest
alternatives are 179cm and 181cm, which tells us what the tolerance is.
The scale maker needs only to ensure that the uncertainties in the
measurements produced by the scale are within a few millimetres, and
typically only within the range 1-2.5m. By comparison, when the
machinist cuts the aluminum alloy bar to length at 180cm, the
uncertainty may be within 0.02 mm. The point is, as John observes, that
the 'quantity value' "180cm" does not, in fact, refer to the same
"measurement" in both cases. "180 cm" means 1.8 times the particular
measurement that is the SI metre, with some uncertainty, but the
uncertainties in these two cases, and the "traces" for them, are
significantly different. We use the value "180cm" in practice, as if
there were a 'quantity magnitude' to which it refers. It is not clear
to me that the VIM agrees that there is one. According to the VIM,
"180cm" refers to a part of a measurement -- the nominal value; and it
is only meaningful in the context of that measurement. Put in the terms
I was using earlier, "180cm" refers to an equivalence class of height
measures using medical office scales, and to a different equivalence
class of mechanical part measures using precision machining equipment of
a certain quality. (06)
-Ed (07)
--
Edward J. Barkmeyer Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263 Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263 FAX: +1 301-975-4694 (08)
"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority." (09)
_________________________________________________________________
Message Archives: http://ontolog.cim3.net/forum/uom-ontology-std/
Subscribe: mailto:uom-ontology-std-join@xxxxxxxxxxxxxxxx
Config/Unsubscribe: http://ontolog.cim3.net/mailman/listinfo/uom-ontology-std/
Shared Files: http://ontolog.cim3.net/file/work/UoM/
Wiki: http://ontolog.cim3.net/cgi-bin/wiki.pl?UoM_Ontology_Standard (010)
|