Integrating Defeaters into Subjective Logic-Based Quantitative Assurance Arguments

A safety assurance argument is a structured reasoning process used to demonstrate that a system meets certain desired safety properties. The argument typically includes claims about the system, evidence supporting those claims, and a clear, logical connection between the evidence and the claims. A critical step in this process is the evaluation of confidence in the argument. To address this step, a range of qualitative and quantitative methods have been proposed. In the qualitative case, defeaters have been used as a dialectical means to challenge nodes in an argument. The presence of defeaters in an assurance argument may highlight reasoning or knowledge gaps, significantly undermining confidence in the argument's validity. However, it is not clear how defeaters can be incorporated into quantitative methods. In this paper, we formalize the notion of defeaters and demonstrate how Subjective Logic can be used to propagate belief, disbelief, and uncertainty within a quantitative assurance argument when these defeaters are present. As a result, this approach enhances the reliability of the argument, allowing for a more rigorous evaluation of safety in complex systems.