[SystemSafety] Does "reliable" mean "safe" and or "secure" or neither?

[Michael J. Pont]

Mike,

We can choose to define any terms that we use in a particular domain in any
way that we like.  

Where these terms conflict with general use of the same word, we make it
more likely that people reading the documents - etc - will be confused (in
my view).  Confusion tends to result in Bad Things Happening in this
business.  We (therefore) want to avoid confusion.

Two seconds on Google gives me this definition of reliability:

"Definition of reliability. 1 : the quality or state of being reliable. 2 :
the extent to which an experiment, test, or measuring procedure yields the
same results on repeated trials."

I see "2" as the useful (and general) definition here.  It may even be
compatible with O'Conner.

My issue is that software (whether good or bad) doesn't change over time.
Talking about "software reliability" (therefore) doesn't make sense and is
likely to lead to confusion.

(Hardware reliability is fine; System reliability is also fine, where System
= Hardware + Software.)

---

While I'm here, "Reliability" isn't my only source of concern ...

People like to say things like:

"Faults may give rise to errors which may give rise to failures".

This usage is common in this domain but isn't (in my view) very helpful,
because "fault" and "error" mean much the same thing to most people (at
least those working in my variety of English).

Again, we can define these terms as we see fit but - if our goal is to avoid
confusion - this doesn't seem to be the best starting point ...

Michael.

-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Mike Ellims
Sent: 24 April 2016 13:23
To: 'Coq, Thierry' <Thierry.Coq at dnvgl.com>; 'The System Safety List'
<systemsafety at lists.techfak.uni-bielefeld.de>
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?

> as software does not "fatigue" or randomly "break" the way hardware 
> does

Reliability engineering is much more than just fatigue or randomly breaks,
it encompasses everything about the control of variation and error in a
system, most commonly using statistical methods.

BS 4478 defines reliability as "The ability of an item to perform a required
function under stated conditions for a stated period of time".

O'Conner states that "reliability is usually concerned with failures in the
time domain. This distinctions marks the difference between traditional
quality control and reliability engineering".

He goes on to list a number of reasons why a failure may occur as follows
(abridged)

- design: which may be inadequate
- overstress: i.e. analysis of condition was incomplete and/or incorrect
- variation: which includes manufacturing variation
- wear out, which is what everyone seems to think of...
- error, such as errors in specification


Henley and Kumamoto give a potted history of the development of reliability
engineering and track its roots to work done by Lusser on the V2 (V1?)
missile which was spectacularly unreliable at first. But obviously not a
wear out issue...

This is of course separate from whether in any context reliability is useful
concept e.g. software. You can obviously measure the reliability of
Windows98 is terms of mean time between crashes (failures in time) likewise
you can measure the reliability of Linux in the same manner. Whether that
has any deep meaning aside from the fact that it shows Windows98 to be pants
compared to Linux for some distribution of uses/input/outputs a different
question.

Cheers.

-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Coq, Thierry
Sent: 24 April 2016 07:24
To: The System Safety List
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?

Hi
I find this list hugely informative. In particular, I find PBL's posts
factual, useful, interesting and intriguing.  As well as the many other
debaters who agree with him or challenge him. As it should be. I wish to
express my gratitude to all debaters.

However, this last exchange seems to me a debate on authority.
On our left, we have DO-178. B now C.
On our right, we have IEC, IEEE, Musa, etc.

To go further, it is plain fact that the aeronautics industry has
demonstrated it doesn't need "software reliability" to deliver highly
reliable automated systems, or systems of systems.
It seems evident with the knowledge we have of the aeronautics success that
in order to use "software reliability" in other industries, or in
aeronautics, there needs to be a clear use case where the value of "software
reliability" is demonstrated, compared to other methods or techniques, in
order to apply "software reliability". The analogy of software and hardware
does not seem valid, as software does not "fatigue" or randomly "break" the
way hardware does, which is the basis for all probabilistic reliability
theories for hardware. The analogy that does seem valid between hardware and
software is the presence of systematic faults, in design, manufacturing,
installation, testing, misuse, etc. Which in hardware also does not have a
probability number. Formal methods can be used to identify such systematic
faults in software. If one can be found, then a test environment can be
devised in which the software will fail 100% of the time. Random hardware
faults do not behave like t  hat.

Best regards,
Thierry Coq
The opinions reflected here are my own and are not necessarily those of my
employer

-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Peter Bernard Ladkin
Sent: dimanche 24 avril 2016 03:27
To: The System Safety List
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?

On 2016-04-23 19:43 , Nick Tudor wrote:
> DO-178C

In the absence of a complete sentence, let me suggest one.

---- DO178C sees no need to assign any meaning to the term "software
reliability".

It's fine for some industry consortium to find it has no use for a specific
concept. RTCA likely has no use for the notion of a cup of tea, either
(BS6008). But that doesn't mean it makes any sense to argue that there isn't
any such thing as a cup of tea.

PBL

Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld,
33594 Bielefeld, Germany Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de






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