Def Stan 00-55 redux

13/07/2014 — Leave a comment

I’ve put the original Def Stan 00-55 (both parts) onto my resources page for those who are interested in doing a compare and contrast between the old, and the new (whenever it’s RFC is released). I’ll be interested to see whether the standards reluctance to buy into the whole safety by following a process argument is maintained in the next iteration. The problem of arguing from fault density to safety that they allude to also remains, I believe, insurmountable.

The justification of how the SRS development process is expected to deliver SRS of the required safety integrity level, mainly on the basis of the performance of the process on previous projects, is covered in 7.4 and annex E. However, in general the process used is a very weak predictor of the safety integrity level attained in a particular case, because of the variability from project to project. Instrumentation of the process to obtain repeatable data is difficult and enormously expensive, and capturing the important human factors aspects is still an active research area. Furthermore, even very high quality processes only predict the fault density of the software, and the problem of predicting safety integrity from fault density is insurmountable at the time of writing (unless it is possible to argue for zero faults).

Def Stan 00-55 Issue 2 Part 2 Cl. 7.3.1

Just as an aside, the original release of Def Stan 00-56 is also worth a look as it contains the methodology for the assignment of safety integrity levels. Basically for a single function or N>1 non-independent functions the SIL assigned to the function(s) is derived from the worst credible accident severity (much like DO-178). In the case of N>1 independent functions, one of these functions gets a SIL based on severity but the remainder have a SIL rating apportioned to them based on risk criteria. From which you can infer that the authors, just like the aviation community were rather mistrustful of using estimates of probability in assuring a first line of defence. 🙂

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