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

From: Peter Bishop < >
Date: Fri, 28 Jun 2013 11:55:16 +0100

As a slight aside, in the past I did some work on characterising the input profile (i.e. the usage environment) in terms of the coverage in different branches of program code

ref: Bishop, P.G. “Rescaling Reliability Bounds for a New Operational Profile”, International Symposium on Software Testing and Analysis (ISSTA 2002), vol. 27 (4), pp. 180-190,2002

If the assumptions made are valid, you can get a massive increase in failure rate if you switch to a new, radically different, profile. On the other hand, if you deliberately test to get "fair" coverage, the rescaled failure rate can be pretty insensitive to changes in profile (even drastic ones).

Obviously, such tests is not possible from operating experience, but use in different profiles gets a bit closer to that ideal

Peter Bishop
Adelard LLP

Peter Bernard Ladkin wrote:
> On 6/27/13 4:23 PM, Nancy Leveson wrote:

>> Someone [Metthew Squair] wrote:
>> > I've been thinking about Peter's example a good deal, the developer 
>> seems to me to have made an
>> > implicit assumption that one can use a statistical argument based on 
>> successful hours run to justify
>> > the safety of the software.
>> And Peter responded:
>> > It is not an assumption. It is a well-rehearsed statistical argument 
>> with a few decades of
>> > universal acceptance, as well as various successful applications in 
>> the assessment of emergency
>> > systems in certain English nuclear power plants.
>> "Well-rehearsed statistical arguments with a few decades of universal 
>> acceptance" are not proof.
>> They are only well-rehearsed arguments. Saying something multiple 
>> times is not a proof.

> What an odd comment, if I have understood it.
> Following:
> 1. One can perform a statistical evaluation of executing SW, based on
> successful hours run, and sometimes use such an evaluation to justify
> one's level of confidence in safety properties of the software;
> 2. This is not an assumption, but a mathematically well-established fact;
> 3. It is, however, of limited application, and the explicit assumptions
> under which one can use it mostly serve to make it impractical for use
> with real SW and real systems;
> 4. No, there is no "proof" (meaning: certainty) of anything established
> by using (most) statistically-valid arguments. Such arguments are mostly
> concerned with levels of confidence around 90-95%.
> This is really basic stuff. I don't understand why anyone would want to
> quibble with any of it.
>> I agree with the original commenter about the implicit assumption, 
>> which the Ariane 5 case disproves
>> (as well as dozens of others).

> Ariane has to do with using SW proven reliable in one environment and
> using it in another environment with input parameters whose distribution
> intersects that of the previous use *in the null set*. It violates one
> of the main conditions of the most common method for statistical
> evaluation of SW to which I refer in Point 1 above. I don't see anything
> in that method that it "disproves". Neither do I understand why you're
> confused about that.
>> Perhaps the reason why software reliability modeling still has pretty 
>> poor performance after at
>> least 40 years of very bright people trying to get it to work is that 
>> the assumptions underlying it
>> are not true.

> To my mind, the reason why it doesn't have more application is that you
> have to do a lot of hard work and have a lot of hard data to make a
> limited inference, and the hard data is mostly not there in most cases.
> Also, as evinced by much of the discussion around such matters, many
> engineers (and not only engineers) are not familiar with reasoning using
> <assertion, confidence> pairs. And people don't use stuff with which
> they are not familiar.
> In this sense, "statistical evaluation" might be the new "formal
> methods". Let's just skip a decade of "don't work/does too work"
> discussion, shall we? I'll have better things to do in old age, such as
> practicing to be a rock star.
>> When someone wrote:
>>  > I don't think that's true,
>> Peter Ladkin wrote:
>>  >>You might like to take that up with, for example, the editorial 
>> board of IEEE TSE.
>> [As a past Editor-in-Chief of IEEE TSE, I can assure you that the 
>> entire editorial board does not
>> read and vet the papers, in fact, I was lucky if one editor actually 
>> read the paper. Are you
>> suggesting that anything that is published should automatically be 
>> accepted as truth? That nothing
>> incorrect is ever published?]

> No, none of that, obviously.
> Prof. Peter Bernard Ladkin, Faculty of Technology, University of
> Bielefeld, 33594 Bielefeld, Germany
> Tel+msg +49 (0)521 880 7319
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systemsafety_at_xxxxxx Received on Fri Jun 28 2013 - 12:55:31 CEST

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