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From: mct@praxis.co.uk (Martyn Thomas)
Newsgroups: comp.software-eng
Subject: Re: Overengineering Software
Message-ID: <1991Apr30.150916.1520@praxis.co.uk>
Date: 30 Apr 91 15:09:16 GMT
References: <1828@qusunita.queensu.CA> <482@data.UUCP>
Organization: Praxis, Bath, U.K.
Lines: 37

kend@data.UUCP (Ken Dickey) writes:

:skill@qucis.queensu.CA (David Skillicorn) writes:
:>... Software is not continuous ...

:Neither is the strength of materials.  Classical example: a large
:number of small dams have been built in this country with wood.  For a
:large dam (e.g. Hoover), you don't add more wood.  The scale requires
:the use of different materials.

There is an important point which may be being missed here.

The behaviour of materials is (close to) continuous, up to the point where
failure occurs. For example, a beam will bend to the point where it breaks.
This means that measurements of the behaviour of materials can be
interpolated. Extrapolation is more risky.

Software behaviour cannot even be interpolated. Consider a weighing machine.
If it is a balance beam, or an extending spring, it can be calibrated and
its accuracy determined by spot measurements and interpolation. If it is
digital, spot measurements only tell you about the accuracy at those exact
weights (and arguably, not much about that). It is perfectly possible that
values between the spot weights will be arbitrarily inaccurate, and only
analysis of the entire system can tell you more. That's one of the penalties
of digital systems.

So how do you certify digital weighing machines? By "engineering judgement",
I guess.

"When they talk about engineering judgement, I know they're just going to
make up numbers." (Richard Feynmann, writing about the Challenger Inquiry,
in 'What do you care what other people think?')


-- 
Martyn Thomas, Praxis plc, 20 Manvers Street, Bath BA1 1PX UK.
Tel:	+44-225-444700.   Email:   mct@praxis.co.uk