Paola Dimaio wrote: (01)
>>So bridges stand because we have a certain amount of useful "knowledge" and
>>they fail because we are not omniscient.
>
> They also fail when our models do not accommodate for interaction and
> change and the immensity of external factors that, in the real world,
> play such a big role. (02)
Yes. That is what I meant by not being omniscient. If we overlook a factor
that plays a big role, then when that factor comes into play, the bridge
fails. Such was the fate of the bridge in Seattle. (03)
> Predicting change can only be done with limited expectation, however
> it is wise to
> have an 'x' factor in our equations (uncertainty) and design for some
> preventive failover mechanism, instead of assuming that our models
> because they are so 'finite' are perfect (04)
I don't understand how to build an X factor to protect from a beast you have
never seen. Engineers build in "margin for error" factors, to accommodate
less than ideal materials, cumulative errors of approximation in estimating
impacts of factors they understand, and unexpected usage behaviors that are
only somewhat outside conventional guidelines. (05)
Most cases of failure involve shoddy material or poor/dangerous design. And
those cases can be described as simple malpractice -- deliberately failing to
have the expertise and materials to do the job adequately. (06)
But the surprise failures are those that involve a factor that was not
considered at all, and not commonly considered in the trade. How do you build
an "X factor" defense for that? We do the analysis of these failures in order
to learn from them! And the approach is pure Sherlock Holmes: When you have
eliminated the impossible, whatever is left, however unlikely, must be the
explanation. The scientific requirement, however, is then to show that that
hypothesis leads to the result that was observed. The end is advancement of
our knowledge, not a better "X factor". (07)
In H.G. Wells War of the Worlds, the Martians were not defeated by unexpected
Earth technologies or climate or radiation, or anything they were able to
observe and measure from space; they were defeated by viruses. (And of
course, Michael Crichton's Andromeda Strain is based on the reverse.) It is
that kind of X factor that brings down the bridge built by experts. (08)
And bridge building has been known to advance medical knowledge. Roebling and
his crew learned a lot about "the bends" (absorption of nitrogen in the blood)
in building the Brooklyn Bridge. That X factor killed him before the bridge
was finished. (09)
> The stability factor in a model can only be a temporary , and must be
> balanced with the 'uncertainty' factors at application stage (010)
I think we agree here. A model is only as good as our knowledge at the time
we built it. As we learn more, some parts of that model may change. But we
can only deal with "uncertainties" that result from factors we know about but
can't predict well, or factors that we understand but can only calculate with
some margin for error. We cannot accommodate factors we know nothing about,
or do not understand. Trying to do that produces X factor models like the
four humours and phlogiston. (011)
-Ed (012)
--
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 (013)
"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority." (014)
_________________________________________________________________
Message Archives: http://ontolog.cim3.net/forum/ontolog-forum/
Subscribe/Config: http://ontolog.cim3.net/mailman/listinfo/ontolog-forum/
Unsubscribe: mailto:ontolog-forum-leave@xxxxxxxxxxxxxxxx
Shared Files: http://ontolog.cim3.net/file/
Community Wiki: http://ontolog.cim3.net/wiki/
To Post: mailto:ontolog-forum@xxxxxxxxxxxxxxxx (015)
|