Formalism and judgement in assurance cases

by Lorenzo Strigini. A position statement for the Workshop on "Assurance Cases: Best Practices, Possible Obstacles, and Future Opportunities", held at DSN 2004, International Conference on Dependable Systems and Networks, Florence, Italy, 2004.

This position paper deals with the tension between the desire for sound and auditable assurance cases and the current ubiquitous reliance on expert judgement. I believe that the use of expert judgement, though inevitable, needs to be much more cautious and disciplined than it usually is. The idea of assurance "cases" owes its appeal to an awareness that all too often critical decisions are made in ways that are difficult to justify or even to explain, leaving the doubt (for the decision makers as well as other interested parties) that the decision may be unsound. By building a well-structured "case" we would wish to allow proper scrutiny of the evidence and assumptions used, and of the arguments that link them to support a decision. An obstacle to achieving this goal is the important role that expert judgement plays in much decision making. The purpose of an assurance case is to a large extent to redirect dependence on judgement to issues on which we can trust this judgement; I doubt that this is done effectively in current practice. Making arguments explicit and if possible mathematically formal is one of the defences, yet formalism does not solve all problems and even creates some of its own. I believe that further progress must depend on better use of the knowledge produced by other disciplines about the cognitive, cultural and organizational factors that affect the production and use of assurance cases, and on studying the successes and failures of assurance cases.

Full text in pdf format.

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