[SystemSafety] Fault, Failure and Reliability Again (short)

[C. Michael Holloway]

On 3/3/15 5:03 AM, Nick Tudor wrote:
> Reliability models for software is still not recognised in DO-178C and 
> this means it has not been recognised for over 25 years.
For those of you without access to DO-178C (2011) the exact wording is 
in 12.3.3 Software Reliability Models:

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

The relevant text in DO-178B (1992) appeared in section 12.3.4.  It was 
a bit longer, beginning with a couple of aspirational sentences, and 
ending with a grant of permission to try:

/During the preparation of this document, methods for estimating the 
post-verification probabilities of software errors were examined.  The 
goal was to develop numerical requirements for such probabilities for 
software in computer-based airborne systems or equipment.  The 
conclusion reached, however, was that the currently available methods do 
not provide results in which confidence can be placed at 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 absence of the aspirational and permission sentences in DO-178C 
could reasonably be interpreted to mean that the level of acceptance of 
software reliability models /decreased/ between 1992 and 2011.

(Note: The quotations from DO-178B and DO-178C are included based on the 
fair use doctrine of US Copyright law.)

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