Assessing Confidence with Assurance 2.0
Robin Bloomfield, John Rushby
TL;DR
Assessing Confidence with Assurance 2.0 presents a rigorous framework for evaluating confidence in assurance cases by separating logical soundness from probabilistic assessments and by systemically handling positive, negative, and residual doubts. It introduces Clarissa/asce as tool support and formalizes five building blocks (concretion, substitution, decomposition, calculation, evidence incorporation) to maintain deductive structure while referencing external theories and models. The paper details how probabilistic valuation can augment but not replace soundness, and it demonstrates how Conservative Bayesian Inference (CBI) links confidence in absence of faults to long-run safety, providing a principled path from evidence to reliability. By integrating defeaters, eliminative argumentation, and dialectical reasoning, Assurance 2.0 aims to deliver indefeasible justification and transparent metacases, enabling graduated, auditable deployment decisions across high-assurance domains.
Abstract
An assurance case is intended to provide justifiable confidence in the truth of its top claim, which typically concerns safety or security. A natural question is then "how much" confidence does the case provide? We argue that confidence cannot be reduced to a single attribute or measurement. Instead, we suggest it should be based on attributes that draw on three different perspectives: positive, negative, and residual doubts. Positive Perspectives consider the extent to which the evidence and overall argument of the case combine to make a positive statement justifying belief in its claims. We set a high bar for justification, requiring it to be indefeasible. The primary positive measure for this is soundness, which interprets the argument as a logical proof. Confidence in evidence can be expressed probabilistically and we use confirmation measures to ensure that the "weight" of evidence crosses some threshold. In addition, probabilities can be aggregated from evidence through the steps of the argument using probability logics to yield what we call probabilistic valuations for the claims. Negative Perspectives record doubts and challenges to the case, typically expressed as defeaters, and their exploration and resolution. Assurance developers must guard against confirmation bias and should vigorously explore potential defeaters as they develop the case, and should record them and their resolution to avoid rework and to aid reviewers. Residual Doubts: the world is uncertain so not all potential defeaters can be resolved. We explore risks and may deem them acceptable or unavoidable. It is crucial however that these judgments are conscious ones and that they are recorded in the assurance case. This report examines the perspectives in detail and indicates how Clarissa, our prototype toolset for Assurance 2.0, assists in their evaluation.