Defeater Cards: Characterizing and Managing Safety Assurance Case Defeaters

2026-06-09

AI Summary

This study addresses the challenge that defeaters in safety arguments—due to their unstructured descriptions and lack of standardized representation—are difficult to review, trace, and reuse. To resolve this, the work proposes Defeater Cards, a novel standardized documentation artifact grounded in the 5W1H framework, offering the first systematic formalism for representing defeaters. The card structure was developed through a literature review and thematic analysis, and its efficacy was validated across multiple case studies spanning diverse domains. Empirical results demonstrate that Defeater Cards effectively expose implicit assumptions and reasoning gaps, substantially enhancing the auditability, traceability, and evolvability of safety arguments. An open-source repository of Defeater Cards is also released to foster knowledge reuse and community-driven collaboration.

Abstract

Safety assurance cases provide structured justifications that safety-critical systems meet their safety requirements. Recently, the notion of defeaters has emerged as a rigorous means of challenging the validity of safety arguments. Examples of defeaters might include overly strict claims, unreliable evidence, or reasoning gaps. However, defeaters remain ad hoc, lack structured support for critical reflection, are inconsistently described, are difficult to review, and lack documentation standards. To address this, we propose Defeater Cards, a new structured documentation artifact for systematically characterizing, reasoning about, and managing defeaters in safety cases. Drawing on a literature survey and thematic analysis, we identify documentation criteria that inform the card's structure, based on the 5W1H framework. Defeater Cards are designed to support informed analysis and evolution, improve traceability and auditability, and enable the reuse of defeater knowledge across systems and product variants. We demonstrate their applicability through two cross-domain case studies, showing how they expose hidden assumptions, surface reasoning gaps, and support ongoing safety assurance case evolution. To support adoption and community reuse, we also release an open-source repository of defeater cards as a baseline upon which researchers and practitioners can build and describe lessons learned.

Problem

• defeaters
• safety assurance cases
• structured documentation
• reasoning gaps
• safety argumentation

Innovation

• Defeater Cards
• Safety Assurance Cases
• 5W1H Framework
• Structured Documentation
• Safety Argumentation