Re: [SystemSafety] RE : Qualifying SW as "proven in use"

From: Peter Bishop < >
Date: Thu, 27 Jun 2013 13:12:53 +0100

I agree with John that caution is needed While stochastic behaviour of the inputs means software failures are also stochastic, the big uncertainty is what happens in a new environment with a new stochastic behaviour. Showing environment X is the "same" as environment Y is pretty tricky and it is not clear if anything can be claimed if the environments are different.

SPRIGGS, John J wrote:
> When Peter's contribution arrived, I was drafting a reply to
> Matthew's posting that also justified the use of statistical
> techniques where failures are systematic. I will not bother with
> that justification now, but I will add a bit of advice for Bertrand's
> postulated safety-engineering rôle-holder.
> Once you have established a quantitative model, it is easy to
> continue working in terms of the numbers and disregard the, perhaps
> tenuous, correspondence with the real world. This is similar to the
> situation with qualitative models where one can be lulled into
> working in terms of a 'feeling', and not acknowledging real-world
> complexity.
> Do not present analysis results with many 'significant figures' and
> do not use them as the sole focus of assurance arguments. Rather,
> use predicted failure rates and associated analyses to support your
> assurance arguments, be they for reliability or for safety.
> Statistical methods are used for quantifying equipment and system
> reliability through the analysis of failure data. Due to the high
> levels of uncertainty involved, these analyses do not offer the level
> of precision that the engineering user may expect. Treating these
> highly-uncertain numbers in the same way as precise measurements is
> unsound and is likely to lead to false conclusions.
> John
> Usual disclaimer about my opinions not being those of my employers,
> clients, et alia -----Original Message----- From:
> systemsafety-bounces_at_xxxxxx > [mailto:systemsafety-bounces_at_xxxxxx > Peter Bernard Ladkin Sent: 27 June 2013 12:35 To: Matthew Squair Cc:
> systemsafety_at_xxxxxx > : Qualifying SW as "proven in use"
> <snip>


Peter Bishop
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Adelard LLP
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Received on Thu Jun 27 2013 - 14:13:09 CEST

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