[SystemSafety] Systematic and random error in systems
Olwen Morgan
olwen at phaedsys.com
Sun Nov 4 17:59:30 CET 2018
All,
Picking up on something Martyn Thomas said a little way back;
In metrology, random error is that kind of error that causes repeated
measurements of a quantity to be dispersed about the true value. A
systematic error is the kind of error that causes repeated measurements
of a quantity to be displaced from the true value. Both kinds of errors
can coexist in a single measuring system.
Now, we know that the effects of software errors are systematic rather
than random. Nevertheless, it is very easy to write, say as embedded
code for a measuring instrument, software whose behaviour produces
measurement errors that are indistinguishable by *tractable* experiment
from random errors. This can be done by adding a pseudorandom quantity
to the measured value before giving it as the output. If you choose the
parameters of the pseudorandom number generator (PRANG) used, then you
can make the output value of a measurement have virtually any form of
distribution you like. Moreover, the only way to detect by experiment
that the error is systematic rather than random is to run the PRANG
through its whole cycle of values at least twice, so that the
periodicity shows up.
I'm not suggesting that people do this but it is, at least to me, a
striking example of how careful we have to be in adapting the principles
of physical measurement to software used in measuring instruments.
Olwen
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