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

From: SPRIGGS, John J < >
Date: Thu, 27 Jun 2013 12:59:38 +0000


After saying in RTCA/DO-178C sub-section 12.3.3, quoted below, that "defection detection rate" is inadmissible, sub-section 12.3.4 allows use of "Actual error rates in the product service history", which I assume is the same as defect detection rate (assuming they fix the ones they find). Or does 12.3.3 mean prediction of the rate of detection of defects?

John
From: systemsafety-bounces_at_xxxxxx Sent: 27 June 2013 13:51
To: systemsafety_at_xxxxxx Subject: Re: [SystemSafety] RE : Qualifying SW as "proven in use"

The full DO-178C text in the section 12.3.3. 'Software Reliability Models' is as follows:

Many methods for predicting software reliability based on developmental metrics have been published, for example, software structure, defection detection rate, etc. This document does not provide guidance for those types of methods, because at the time of writing, currently available methods did not provide results in which confidence can be placed.

This text is a simplification of the text from DO-178B section 12.3.4 'Software Reliability Models':

During the preparation of this document, methods for estimating the post-verification probabilities of software errors were examined. The goal as to develop numerical requirements for such probabilities for software in computer-based airborne systems or equipment. The conclusion reached, however, was the currently available methods do not provide results in which confidence can be places to the level required for this purpose. Hence, this document does not provide guidance for software error rates. If the applicant proposes to use software reliability models for certification credit, rationale for the model should be included in the Plan for Software Aspects of Certification, and agreed with by the certification authority.

The removal in C of the "If the applicant ..." sentence could be interpreted as implying even less confidence today in the utility of software reliability models than was expressed in 1992.

--

C. Michael Holloway

Disclaimer1: My opinions are mine alone. Give neither blame nor credit to my employer for them.

Disclaimer2: The quotations from DO-178B/C are justified by the fair-use exception to copyright protection.
On 6/27/13 7:40 AM, Nick Tudor wrote:
There is adequate guidance within DO178C on the evidential requirements for previously developed software.  My opinion is that it basically gives the hint that it is very difficult and that anyone considering doing it for anything above DAL C should re-consider.  Also note that there is a statement in DO178C regarding software 'reliability' as follows: "Many methods for predicting software reliability based on developmental metrics (for example, software structure, defect detection rate, etc.) have been published. This document does not provide guidance for those types of methods, because at the time of writing, currently available methods did not provide results in which confidence can be placed."

Cheers

Nick Tudor

On 27 June 2013 10:24, Matthew Squair <mattsquair_at_xxxxxx
Hi Bertrand,

I think that you touch on one of the great problems of 61508 and it's children, that it becomes a distorting prism through which everyone views safety.
There is always a lot one can do from a practical perspective but this is too often obscured by the obsfucation of ISO 61508 artefact, the somewhat reflexive response of COMAH to the Buncefield accident being a case in point (http://wp.me/px0Kp-1BD).

That being said, I do come back to the issue that Peter's original example has crystallised for me, which is that if you accept that random and systematic errors are different, then you should not expect the techniques to characterise one type of failure to automatically apply to the other.

I'll tip my hat a little and add that I do think there's room for a "proven in use" argument as well...

--
Matthew Squair
Email: MattSquair_at_xxxxxx
Mob: +61 488770655<tel:%2B61%20488770655>
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On Thursday, 27 June 2013 at 6:07 PM, RICQUE Bertrand (SAGEM DEFENSE SECURITE) wrote:

Let's put Peter's example in an more industrial and less academic frame.



You are an instrumentist and automation technician. Your school background is bachelor + 2 years of professional training more or less on how to configure a PLC with graphical languages and to install pressure transmitters on pipes. You are working since ten years in a chemical plant. You know how to design a PID control loop for a level control and how to sequence bits of automatic control on your processes. Your boss recently sent you to a 3 to 5 days 61508 training session and now you are known in your company as a safety engineer. There is a plan for a revamping or an aextension of a part of your plant and your boss asks you to take care of all the safety systems because there are these new standards the plant has to comply with. All your usual suppliers come to propose you "certified safety equipments" but your boss refuse to add new items in the plant because of cost of adding new references in the stocks. So you are request to investigate how the existing equipment and architectures can be reused. Because this only a fictive choice, this has to be understood as: you are requested to find a way to demonstrate that the reused equipment complies to requirements equivalent to DAL B in avionics.



May we add that you don't understand more than one word among five from what is debated in this forum, not even speaking about the concepts that go behind. In your mind, if an application involving a transmitter, a valve, a PLC housing a piece of software, that yourself or anybody else has coded five years ago, has never failed (failed not being clearly defined), there is no reason to believe that it might fail in the future if you apply it again in a rather similar way (similar not being better defined than failed).



That's the issue, the context and the background for which we are now writing a standard.



Bertrand RICQUE

Program Manager, Optronics and Defense Division



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From: Matthew Squair [mailto:mattsquair_at_xxxxxx
Sent: Thursday, June 27, 2013 9:19 AM
To: RICQUE Bertrand (SAGEM DEFENSE SECURITE)
Cc: systemsafety_at_xxxxxx
Subject: Re: [SystemSafety] RE : Qualifying SW as "proven in use"



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 sucessful hours run to justify the safety of the software.



I don't think that's true, in fact I'd go further and say that whether you operate for a thousand hours or a million hours has no bearing on demonstrating software safety, because what we're interested in are systematic failures rather than random ones.



Example, I have a piece of software and (despite my best efforts) there's a latent fatal fault within it, however testing hasn't discovered it and I'm also in luck in that the operating environment is sufficiently close to the test environment that the fault is not triggered in the operating environment. Now I could run the system for one, one hundred or a thousand years in that operating environment and I wouldn't see a problem. So according to the statistical treatment the software is safe, even with a fatal flaw isn't it?



So logically if the number of hours you run in service in a particular environment has nothing to do with proving the safety of software, why couldn't I say that after one hundred hours the software was 'proven in use', for that specific environment. Why not one hour?



In Peter's example the number of hours run on the original software version could have been one, or ten million and there still would have been the same end result, e.g a failure when put into a new operational context. In other words one hour of operations has as much weight as one thousand (in the same environment).



We use hours as a measure of exposure when we test for random failures because they're well, random, but systematic failures aren't random so I don't think that using hours actually works. So what's the 'unit of exposure' that is valid for systematic failures?



Another question, say I have developed a piece of software, it's now running in three quite different operating environments, in terms of evidence of 'safety' would I weight 300 hours of operation in a single environment the same as 100 hours from each of these different environments? If so why?

















On Tue, Jun 18, 2013 at 6:41 PM, RICQUE Bertrand (SAGEM DEFENSE SECURITE) <bertrand.ricque_at_xxxxxx

Hi Peter,

Let's start some miles behind.

There are 2 ways to claim that anything is compliant to 61508:
1 - Compliance. Yes, I know, it looks stupid as the way to claim compliance is nor ruled by the standard neither by the market. I can claim it on the ground of my reputation without having to bring any proof. Let's assume: on the basis of analysis made by firms having a strong reputation on the market.
2 - Proven use.

Proven in use: what a nice idea.

The standard is about safety. A manufacturer cand esign and put on the market a new equipment and get it "certified " for "compliance" for a given use (restricted operational and functional environment) for a "safety" purpose. Let's say safety critical pupose.

Now let's go to your paper. First of all, you choose the example of a manufacturer, but we must rememeber that the standard adresses anybody. So a user could do it. Let's assume the user has more or less the same problems than the manufacturer.

There are two possibilities for the previous use of the C equipment:
1 - previously used in a safety application with all the associated usual safety requirements (in particular dysfunctional requirements). Then it means obviously that the plant/application was NOT already compliant to 61508. Let's assume this is possible, although I doubt that plenty of users will shout it to the public...
2 - previously used in a non safety application. We can imagine that there are non-safety applications presenting very similar characteristics. I have however some doubts but lets assume it.


>From a manufacturer point of view, it can be interesting to re-use C, most probably parts of C as C-SW, in new products. Honestly, I don't see any manufacturer going to the market saying: hey guys, here is my very old equipment that I have re-packaged and will sell you twice the price for safety applications with a nice SIL 3 stamp. The need is more like: hello, my dear certification company, here is my new product, it made of well known pieces, so please don't charge us too much for the certificate. You can consider C-SW as "proven in use", here are the data.

>From an end-user point of view, (and this is what was in mind of the writers - "was" because it is becoming too demanding, and they are loosing the rules in 61511, pushing BTW 61508 for manufacturers only ...), if one can get rid of case 1, the issue is not to change anything in an existing plant and to, would an inspector ask strange questions, proove that the plant is the safest in the world and complies to any possible regulation. For competent and responsible end-users, the need it to have a framework to properly select equipments on sound basis and not on manufacturers data sheets.
That's the context at business level. It is clear that points 1 and 2 of Martyn, in a non regulated context, will remain a dream for ever... When it goes to technical level: * Concerning manufacturers, in the industries at the origin of the standard, I don't know a single one having a solid process to collect data from the users. Some adjust the calculated MTBF with the return rate for repair in the factories. Concerning electronic equipment, if an expensive piece of hardware is not attached to the electronic board, it is usually thrown to the garbage... Most of them don't knwo at alla where and how are installed the equipement. It is actually the opposite The users are the best organised to gather data (OREDA, EIREDA, etc...). So I think that the issue is very theoretical for manufacturers. * Concerning end-users, before even entering in your considerations (I fully support BTW), and remaining within the concerns of the writers of the standard (shaping a much narrower picture actully): - Most systems operate on demand and, due to the functional characteristics of the applications, experience very few demands (otherwise, it means that the plants are permanently in trouble. This means that the chances that there could be a reasonable quantitative basis to extrapolate anything seem to me very low. - What is happening now in IEC 61511 discussions (with more or less the same persons as the ones at the origin of IEC 61508) is that three parties are building themselves: 1 - the ones (mostly end users) thinking that the concept dosn't apply to "re-use" but only to "already installed". They argue that if it is working properly, it will continue so. They keep a low profile on "re-use" but I suspect them to focus the discussion on "already installed" and use the concept for "re-use" later as it would not be clearly excluded in the standard ... They are also the ones that, rightly, don't want to buy expensive "certified" equipment with a lower performance than equipment they already have in their plant. The issue is real and interesting. These ones are mainly eurpoeans. 2 - the ones (mostly end-users) who want to re-use equipment because they want to reduce the number of different equipment in a plant (they are also strong opponents to diversity because of costs). These ones are mainly North American. 3 - the manufacturers are mainly against because they want to sell new equipment and they don't see how to actually "prove in use" as they lack data. The consultants are against because they don't where is the rationale (exactly what your paper is about). I am rather pessimistic about the outcomes in 61511. As far as 61508 is concerned, my opinion, converging more and more with several opinions expressed here is that: * Concerning software, the requirements must obviously be very stringent * This will implicitely so much limit the applicability of the concept that it will become useless for end-users and PLCs, * May-be it will be applicable for very tiny parts of firmware for manufacturers * It will de-facto create an insconsistency with 61511 if 61511 doesn't align on 61508 requirements Bertrand Ricque ________________________________________ Date d'envoi : lundi 17 juin 2013 12:32 À : systemsafety_at_xxxxxx Objet : [SystemSafety] Qualifying SW as "proven in use" Folks, there is a significant question how SW can be qualified as "proven in use" according to IEC 61508:2010. There is a judgement in some quarters (notably the German national committee) that the criteria in IEC 61508:2010 are inappropriate. I think it wouldn't be out of place to say that many in the IEC 61508 Maintenance Teams find the current criteria unsatisfactory in one way or another. We in Germany have been discussing the issue and possible solutions for a couple of years, and recently the discussion has gone international. There seems to be a general feeling that qualifying SW statistically via the approach given by the exponential failure model is not practical, because the data requirements are overwhelming - it is regarded by most as implausible that companies will have the requisite data to the requisite quality even for SIL 2. But even if you qualify your SW for SIL 2 or higher without such data, then at some point some data will exist and people use such data as evidence that the original assessment was accurate. But what sort of evidence does it offer? The answer is probably a lot less than you might be convinced it does. There seems to me to be a lack of examples where things can go wrong - at least a lack of examples specifically adapted to assessments according to IEC 61508:2010. So I wrote one up - fictitious but I hope still persuasive - to illustrate what (some of) the assurance issues are. I hope it can aid the debate. http://www.rvs.uni-bielefeld.de/publications/WhitePapers/LadkinPiUessay20130614.pdf PBL -- Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Tel+msg +49 (0)521 880 7319<tel:%2B49%20%280%29521%20880%207319> www.rvs.uni-bielefeld.de> _______________________________________________ The System Safety Mailing List systemsafety_at_xxxxxx # " Ce courriel et les documents qui lui sont joints peuvent contenir des informations confidentielles ou ayant un caractère privé. 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