Re: [SystemSafety] Safety Cases

From: Andrew Rae < >
Date: Mon, 10 Feb 2014 00:16:01 +0000


Tracy,
The important point is that "done" and "claimed" are different things, not synonyms as you imply. Activities that are very good at the "done" are not necessarily very useful for the "claimed" and vice versa.

In particular, a lot of activities that go into making a safe design are only indirectly evidence that the design is safe. In essence, they are evidence that've tried hard, not that you've achieved anything. This is why we distinguish between "process" and "product" evidence. One of the advantages of explicit safety cases is they force you to consider exactly what your evidence shows or doesn't show.

Contrawise, some activities which are used a lot to generate evidence are only indirectly helpful at making a design safer. A lot of quantitative analysis goes into this basket. Only if it reveals issues that are addressed through changes to design or operation can quantitative analysis actually directly improve safety. Otherwise it is evidence without improvement.

Drew

My system safety podcast: http://disastercast.co.uk My phone number: +44 (0) 7783 446 814
University of York disclaimer:
http://www.york.ac.uk/docs/disclaimer/email.htm

On 9 February 2014 22:26, Tracy White <tracyinoz_at_xxxxxx

>
> [Andrew Rae Stated]
>
>
>
> *(Note: not all safety activities are about evidence. Most of them are
> about getting the design right so that there _aren't_ safety problems that
> need to be revealed).*
>
>
>
> I completely agree that 'getting the design right' is an important element
> of any assurance argument but I disagree that it can be done (claimed)
> without providing 'evidence'. If you think you can claim that you got the
> 'design right', then you must have done something to achieve that and for
> those efforts there will be evidence.
>
>
> Regards, Tracy
>
> ------------------------------
> On 7 Feb 2014, at 23:24, Andrew Rae <andrew.rae_at_xxxxxx >
> If I can slightly reframe from Martin's points, the real problem is asking
> these questions in the negative. If the system _didn't_ have the properties
> it needs, what activities or tests would be adequate to reveal the
> problems?
>
> Whenever there is a focus on providing evidence that something is true,
> this is antithetical to a proper search for evidence that contradicts.
> As Martin points out, most evidence is not fully adequate to show that
> properties are true. The best we can do is selecting evidence that would
> have a good chance of revealing that the properties were not true.
>
> (Note: not all safety activities are about evidence. Most of them are
> about getting the design right so that there _aren't_ safety problems that
> need to be revealed).
>
> Simple question for the list (not directly related to safety cases):
>
> How often have you seen a safety analysis that was:
> a) Conducted for a completed or near completed design
> b) Revealed that the design was insufficiently safe
> c) Resulted in the design being corrected in a way that addressed the
> revealed problem(s)
>
> Supplementary question:
> What was the activity?
>
> [Not so hidden motive for asking, just so the question doesn't look like a
> trap - I've seen a lot of QRA type analysis that meets (a), but the only
> times I've seen (b) and (c) follow on are when the analysis is reviewed,
> not when the analysis is conducted]
>
> Drew
>
>
>
>
> 1 What properties does the system need to have in order for it to be
> adequately dependable for its intended use? (and how do you know that these
> properties will be adequate?)
> 2 What evidence would be adequate to show that it had these properties?
> 3 It it practical to aquire that evidence and, if not, what is the
> strongest related property for which it would be practical to provide
> strong evidence that the property was true?
> 4 What are we going to do about the gap between 1 and 3?
>
> My system safety podcast: http://disastercast.co.uk
> My phone number: +44 (0) 7783 446 814
> University of York disclaimer:
> http://www.york.ac.uk/docs/disclaimer/email.htm
>
>
> On 7 February 2014 12:05, RICQUE Bertrand (SAGEM DEFENSE SECURITE) <
> bertrand.ricque_at_xxxxxx >
>> It seems to me that at the end of the reasoning, the standard xyz (e.g.
>> IEC 61508) requests some work to be done available in documents (whatever
>> the name). Standard xyz contains (strong) requirements on 1 and (weaker)
>> requirements on 2 but at least requirements on the means and methods to
>> achieve 1.
>>
>>
>>
>> It looks circular.
>>
>>
>>
>> In the understanding of stakeholders being compliant to standard xyz
>> means not doing a lot of engineering stuff that is unfortunately explicit
>> or implicit in the standard xyz. But most often they even never read it.
>> This is also an explanation about the observed gap in the industry.
>>
>>
>>
>> Bertrand Ricque
>>
>> Program Manager
>>
>> Optronics and Defence Division
>>
>> Sights Program
>>
>> Mob : +33 6 87 47 84 64
>>
>> Tel : +33 1 59 11 96 82
>>
>> Bertrand.ricque_at_xxxxxx >>
>>
>>
>>
>>
>>
>>
>> *From:* systemsafety-bounces_at_xxxxxx >> systemsafety-bounces_at_xxxxxx >> Thomas
>> *Sent:* Friday, February 07, 2014 12:16 PM
>> *To:* systemsafety_at_xxxxxx >> *Subject:* [SystemSafety] Safety Cases
>>
>>
>>
>> In the National Academies / CSTB Report *Software for Dependable
>> Systems: Sufficient Evidence?* (
>> http://sites.nationalacademies.org/cstb/CompletedProjects/CSTB_042247)
>> we said that every claim about the properties of a software-based system
>> that made it dependable in its intended application should be stated
>> unambiguously, and that every such claim should be shown to be true through
>> scientifically valid evidence that was made available for expert review.
>>
>> It seems to me that this was a reasonable position, but I recognise that
>> it is a position that cannot be adopted by anyone whose livelihood depends
>> on making claims for which thay have insufficient evidence (or for which no
>> scientifically valid evidence *could* be provided). Unfortunately, much
>> of the safety-related systems industry is in this position (and the same is
>> true, *mutatis mutandis*, for security).
>>
>> It seems to me that some important questions about dependability are
>> these:
>>
>> 1 What properties does the system need to have in order for it to be
>> adequately dependable for its intended use? (and how do you know that these
>> properties will be adequate?)
>> 2 What evidence would be adequate to show that it had these properties?
>> 3 It it practical to aquire that evidence and, if not, what is the
>> strongest related property for which it would be practical to provide
>> strong evidence that the property was true?
>> 4 What are we going to do about the gap between 1 and 3?
>>
>> The usual answer to 4 is "rely on having followed best practice, as
>> described in Standard XYZ". That's an understandable position to take, for
>> practical reasons, but I suggest that professional ingegrity requires that
>> the (customer, regulator or other stakeholder) should be shown the chain of
>> reasoning 1-4 (and the evidence for all the required properties for which
>> strong evidence can be provided) and asked to acknowledge that this is good
>> enough for their purposes.
>>
>> I don't care what you choose to call the document in which this
>> information is given, so long as you don't cause confusion by overloading
>> some name that the industry is using for something else.
>>
>> I might refer to the answers to question 1 as a "goal", if I were trying
>> to be provocative.
>>
>> Martyn
>>
>>
>>
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