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

From: Peter Bernard Ladkin < >
Date: Fri, 28 Jun 2013 12:16:03 +0200

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.

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.

PBL Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Tel+msg +49 (0)521 880 7319

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