Path: utzoo!mnetor!tmsoft!torsqnt!lethe!yunexus!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!mcsun!ukc!reading!minster!mrp
From: mrp@minster.york.ac.uk
Newsgroups: comp.software-eng
Subject: Re: Tolerance
Message-ID: <665834913.1874@minster.york.ac.uk>
Date: 6 Feb 91 10:08:34 GMT
References: <1401@ucl-cs.uucp> <27A9B451.48BF@tct.uucp> <15863.27ad36b6@levels.sait.edu.au> <777@caslon.cs.arizona.edu> <87999@tut.cis.ohio-state.edu> <5614@skye.cs.ed.ac.uk>
Reply-To: mrp@SoftEng.UUCP (mrp)
Organization: Department of Computer Science, University of York, England
Lines: 41

The point being missed so far is when can a system 'make-do' with an
'imprecise/less-accurate/less tolerant/in-exact/imprefect' result.

The driving force behind a system using imprecision is the
time-precision trade off.  This requires that algorithms are written
so the preciseness of the result as it is sampled during the execution
of the task rises as more time is spent executing the task.  Thus when
the task stops the precision of the result is dependent on how much of
the code the task managed to execute.  [see Lin,Swaminathan & Liu,
IEEE RTS Symposium 1987].

Now suppose you have a system dealing with locating objects and to
precisely locate one object takes a task one second.  Assuming you are
using a single processor system and the number of seconds you have
available out-numbers the number of objects to locate, all results
will be as precise as they can be.  Now suppose you have 3 seconds in
which to locate 6 objects.  In 'normal' real-time systems, 3 tasks
would run to completion and return the results and 3 would not be
scheduled at all.  In an imprecise system each task could be run for
1/2 a second, each returning an 'imprecise' result.  These could be in
a form of an area where the object is, rather than an exact location.
For example a result of 'at position x with a precision of 75%' could
be interpreted as 'in a circular area centred on x radius 7.5 meters'.
(Defining metrics for imprecise results is a very tricky problem,
especially in non-numerical domains.)

Depending on the application, having imprecise data on six objects may
be considered more useful than knowing exact locations of 3 objects
and knowing nothing about 3 others.  I would definitely prefer to know
the approximate positions of six other vehicles on the road around me
than know exactly where 3 were and have no idea about the other three.

   - Martin

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| Martin Portman                | Phone: (0904) 432735                      |
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