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Uncertainty and Confidence in Safety Logic

Fulltext:


Authors:

Patrick Graydon

Research group:

Safety-Critical Engineering

Publication Type:

Conference/Workshop Paper

Venue:

31st International System Safety Conference

Publisher:

System Safety Society

Abstract

Reasoning about system safety requires reasoning about confidence in safety claims. For example, DO-178B requires developers to determine the correctness of the worst-case execution time of the software. It is not possible to do this beyond any doubt. Therefore, developers and assessors must consider the limitations of execution time evidence and their effect on the confidence that can be placed in execution time figures, timing analysis results, and claims to have met timing-related software safety requirements. In this paper, we survey and assess existing concepts that might serve as means of describing and reasoning about confidence, including safety integrity levels, probability distributions of failure rates, Bayesian Belief Networks, argument integrity levels, and Baconian probability. We define use cases for confidence in safety cases, prescriptive standards, certification of component- based systems, and the reuse of safety elements both in and out of context. From these use cases, we derive requirements for a confidence framework. We assess existing techniques by discussing what is known about how well each confidence metric meets these requirements. Our results show that no existing confidence metric is ideally suited for all uses. We conclude by discussing implications for future standards and for reuse of safety elements.

Bibtex

@inproceedings{Graydon2994,
author = {Patrick Graydon},
title = {Uncertainty and Confidence in Safety Logic},
month = {August},
year = {2013},
booktitle = {Proceedings of the 31st International System Safety Conference (ISSC)},
publisher = {System Safety Society},
url = {http://www.es.mdu.se/publications/2994-}
}