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

From: SPRIGGS, John J < >
Date: Thu, 27 Jun 2013 11:52:44 +0000


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-----
Sent: 27 June 2013 12:35
To: Matthew Squair
Cc: systemsafety_at_xxxxxx Subject: Re: [SystemSafety] RE : Qualifying SW as "proven in use"

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systemsafety_at_xxxxxx Received on Thu Jun 27 2013 - 13:52:57 CEST

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