Defence Standard 00-56 Issue 4: Towards Evidence-Based Safety Standards

Defence Standard 00-56 Issue 4 is goal-based, and requires system developers to demonstrate how they have achieved safety. To this end, evidence is used to support claims relating to software safety. One of the most subtle questions when constructing a safety argument is the determination of whether the evidence presented is sufficient to assure the safety of the system to the level required. This paper presents a framework for assessing the assurance of evidence and claims. We also present a vocabulary for discussing factors which influence assurance. This framework and vocabulary together enable us to construct and discuss safety arguments for software. Using this framework and vocabulary, we present some sample discussions which demonstrate how the factors influencing assurance can interact.