[SystemSafety] WG: words you cannot use at GM

[Matthew Squair]

>From a systems engineering perspective the word 'acceptable' is a
subjective qualifier, meaning the requirement for something to be
'acceptably safe' is unverifiable as it introduces subjective and
potentially different interpretations.

Likewise terms such as 'reasonably', 'credible' and 'practicable' introduce
subjective interpretation and judgement. Unless you carefully define in a
verifiable sense what each means of course.

I also tend to the school of thought that if you can't verify a requirement
in any credible sense it's not a technical requirement, ergo such concepts
as SFAIRP or ALARP on their own are not true requirements,  nor should they
be introduced into specifications.

Matthew Squair

MIEAust, CPEng
Mob: +61 488770655
Email; Mattsquair at gmail.com
Web: http://criticaluncertainties.com

On 23 May 2014, at 1:18 am, Nancy Leveson <leveson.nancy8 at gmail.com> wrote:

There is nothing wrong with saying "acceptably safe" (of course you need to
define what you mean by "acceptable" or the statement is useless). The
problem is in *defining* "safe" as "acceptably safe" and then simply
declaring that the system is "safe." Safe is not the same as "acceptably
safe with a probability of 10^-9 (or whatever).

Nancy


On Thu, May 22, 2014 at 9:25 AM, <peter.sheppard at uk.transport.bombardier.com
> wrote:

>
> I don't see what is wrong with "acceptably safe".  No system is ever 100%
> safe, thus an expert, professional judgement on what is acceptably safe has
> to be made.
>
> In the UK railways it was agreed that "acceptably safe" for rail workers
> was different to what "acceptably safe" was for passengers (still can't
> call them customers!)
>
> Regards
>
> Peter
>
> Peter Sheppard
> Senior Safety Engineer and Validator
>
> Mobile: +44 7920 247931
>
>
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>
>
> *Peter Bernard Ladkin <ladkin at rvs.uni-bielefeld.de
> <ladkin at rvs.uni-bielefeld.de>>*
> Sent by: systemsafety-bounces at lists.techfak.uni-bielefeld.de
> 22/05/2014 13:46
> To
> systemsafety at lists.techfak.uni-bielefeld.de
> cc
>   Subject
> Re: [SystemSafety] WG: words you cannot use at GM
>
>
>
>
>
>
> On 2014-05-22 14:31 , Nancy Leveson wrote:
> > I think saying that "acceptably safe" is safe is a ridiculous
> definition.
>
> Nevertheless, it is de rigeur in Europe. The UK Health and Safety
> Executive says that is how it
> judges, with "acceptably safe" usually meaning a one in a million elevated
> chance of dying:
> http://www.hse.gov.uk/risk/theory/r2p2.htm
>
> PBL
>
> Prof. Peter Bernard Ladkin, Faculty of Technology, University of
> Bielefeld, 33594 Bielefeld, Germany
> Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de
>
>
>
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-- 
Prof. Nancy Leveson
Aeronautics and Astronautics and Engineering Systems
MIT, Room 33-334
77 Massachusetts Ave.
Cambridge, MA 02142

Telephone: 617-258-0505
Email: leveson at mit.edu
URL: http://sunnyday.mit.edu

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