[SystemSafety] Systematic and random error in systems

[Peter Bernard Ladkin]

Folks,

On 2018-11-06 02:07 , Matthew Squair wrote:
> Another way to view systematic error is that you are not directly measuring the parameter of
> interest but instead some analogue of it. 
The use of the terms "random failure" and "systematic failure" in IEC 61508 is somewhat different
from what you are talking about here.

The reference to the IEV in subclause 3.6.6 could theoretically be followed by looking at
electropedia.org, but in fact Part 191 is not there. Part 192 is, however, and subclause 192-3-10
gives a definition of "systematic failure" which is at variance with that in IEC 61508-4:2010. One
human reason for that is that Part 192 is put together by TC56, a different committee from SC65A
with almost no overlap in members (there is one French member who is in TC56 committees as well as
the 61508 MTs). I worked in TC56 for a brief period in 2011-2013 and left after my attempted
contributions were deprecated by the then-management (quote from the then-Chair: "you are a safety
person and we don't do safety". Despite producing the standards on the key analysis techniques for
safety, namely FTA, FMEA, ETA and Root Cause Analysis. And despite my having worked on software
dependability longer than I have worked on safety).

The term "random" occurs only in the phrase "random hardware failure" (often elided to "random
failure"). It does not correspond to use of the term by probability theorists, statisticians and
risk analysts. It means that your kit breaks.

Continuing, NOTE 4 to subclause 3.6.6 makes it clear (by saying so explicitly!) that a "systematic
failure" is any failure of a safety-related system that is not a random hardware failure.

[begin quote IEC 61508-4:2010 subclauses 3.6.5 and 3.6.6]

3.6.5
random hardware failure

failure, occurring at a random time, which results from one or more of the possible degradation
mechanisms in the hardware

NOTE 1 There are many degradation mechanisms occurring at different rates in different components
and, since manufacturing tolerances cause components to fail due to these mechanisms after different
times in operation, failures of equipment comprising many components occur at predictable rates but
at unpredictable (i.e. random) times.

NOTE 2 A major distinguishing feature between random hardware failures and systematic failures (see
3.6.6), is that system failure rates (or other appropriate measures), arising from random hardware
failures, can be predicted with reasonable accuracy but systematic failures, by their very nature,
cannot be accurately predicted. That is, system failure rates arising from random hardware failures
can be quantified with reasonable accuracy but those arising from systematic failures cannot be
accurately statistically quantified because the events leading to them cannot easily be predicted

3.6.6
systematic failure

failure, related in a deterministic way to a certain cause, which can only be eliminated by a
modification of the design or of the manufacturing process, operational procedures, documentation or
other relevant factors

[IEV 191-04-19]

NOTE 1 Corrective maintenance without modification will usually not eliminate the failure cause.

NOTE 2 A systematic failure can be induced by simulating the failure cause.

NOTE 3 Examples of causes of systematic failures include human error in
- the safety requirements specification;
–– the design, manufacture, installation, operation of the hardware;
–– the design, implementation, etc. of the software.

NOTE 4 In this standard, failures in a safety-related system are categorized as random hardware
failures (see 3.6.5) or systematic failures.

[end quote]

PBL

Prof. Peter Bernard Ladkin, Bielefeld, Germany
MoreInCommon
Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs-bi.de





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