"Validation of ultra-high dependability..." - 20 years on

By Lorenzo Strigini and Bev Littlewood; Safety Systems, Newsletter of the Safety-Critical Systems Club, May 2011

ABSTRACT
In 1990, we submitted a paper to the Communications of the Association for Computing Machinery, with the title "Validation of Ultra-High Dependability for Software-based Systems". The immediate trigger for the discussions that led to that paper were the requirements of failure probability of less than 10-9 per hour, or per cycle, for some safety-critical equipment in civil aircraft. We thought that the then-typical approach to this issue (codified in the DO-178B document) did not inspire confidence. We paraphrased (some people said caricatured) the position taken in DO-178B as "a very low failure probability is required but, since its achievement cannot be proven in practice, some other, insufficient method of certification will be adopted". We also predicted that both this kind of extreme requirements, and the inadequate justification of their satisfaction, would spread to many more systems and industrial sectors, as they have. Back then, different people had different takes on the issue, but our concerns were widely shared. .... This anniversary of the SCSC falls about 20 years later, so it seems a good time to revisit briefly our article and see where the debate about these issues now stands.

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