[SystemSafety] Software Safety Requirements according to IEC 61508

[Matthew Squair]

>So (an interesting but secondary question) why has it persisted?

Availability heuristic perhaps?

Pure '...it was the software your honour' type accidents are pretty rare
and if over a 20 year engineering career you only get say a couple of
thousand hours of hands on experience?

>Why do something that demonstrably brings you nothing, but costs resources?

Because the standard made me? :)

Actually the legal formalism implicit in the wise crack above is one
justification.

But I think it has more to do with the historical approach embodied in such
artefacts as the V model, where we 'validate' the system at the end, ergo
let's 'validate' the safety as well. If all you have is a hammer...


On Sun, May 22, 2016 at 4:10 PM, Peter Bernard Ladkin <
ladkin at rvs.uni-bielefeld.de> wrote:

>
>
> On 2016-05-22 02:59 , Matthew Squair wrote:
> > I think that the 61508 statement regarding validation simply reflects a
> prevalent world view.
>
> I agree with you there. But it was shown, twice, nearly a quarter century
> ago in the literature that
> this world view is inappropriate for systems required to be ultrareliable.
> So (an interesting but
> secondary question) why has it persisted?
>
> > From a Bayesian perspective, for this behaviour to be rational we would
> need to start with a very
> > high prior probability (belief) that our system under test delivers the
> safety function and
> > associated properties.
>
> Yes.
>
> > So yes validating through testing is rational, as long as we have that
> > extreme level of confidence.
>
> No it's not.
>
> If you start out with a strong enough prior, all the testing it is
> feasible to do cannot improve
> that prior ("improve" here means increase the prior reliability estimate,
> that is, decrease the
> estimated failure rate).
>
> Why do something that demonstrably brings you nothing, but costs resources?
>
> 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
>
>
>
>
>
>


-- 
*Matthew Squair*
BEng (Mech) MSysEng
MIEAust CPEng

Mob: +61 488770655
Email: MattSquair at gmail.com
Website: www.criticaluncertainties.com <http://criticaluncertainties.com/>
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