Modeling the probability of failure on demand (pfd) of a 1-out-of-2 system in which one channel is "quasi-perfect". (February 2017)

• Record Type: Journal Article
• Authors: Zhao, Xingyu; Littlewood, Bev; Povyakalo, Andrey; Strigini, Lorenzo; Wright, David
• Is Part Of: Reliability engineering & system safety. Volume 158(2017)
• Page Start: 230
• Page End: 245
• Publication Date: 2017-02
• Subjects: Fault-free software -- Program perfection -- Quasi-perfection -- Probability of perfection -- 1-out-of-2 system reliability -- Software diversity; Reliability (Engineering) -- Periodicals; System safety -- Periodicals; Industrial safety -- Periodicals; Fiabilité -- Périodiques; Sécurité des systèmes -- Périodiques; Sécurité du travail -- Périodiques; 620.00452
• DOI: 10.1016/j.ress.2016.09.002
• Languages: English
• ISSNs: 0951-8320

Abstract

Abstract: Our earlier work proposed ways of overcoming some of the difficulties of lack of independence in reliability modeling of 1-out-of-2 software-based systems. Firstly, it is well known that aleatory independence between the failures of two channels A and B cannot be assumed, so system pfd is not a simple product of channel pfd s. However, it has been shown that the probability of system failure can be bounded conservatively by a simple product of pfd A and pnp B (probability not perfect) in those special cases where channel B is sufficiently simple to be possibly perfect. Whilst this "solves" the problem of aleatory dependence, the issue of epistemic dependence remains: An assessor's beliefs about unknown pfd A and pnp B will not have them independent. Recent work has partially overcome this problem by requiring only marginal beliefs – at the price of further conservatism. Here we generalize these results. Instead of "perfection" we introduce the notion of "quasi-perfection": a small pfd practically equivalent to perfection (e.g. yielding very small chance of failure in the entire life of a fleet of systems). We present a conservative argument supporting claims about system pfd . We propose further work, e.g. to conduct "what if?" calculations to understand exactly how conservative our approach might be in practice, and suggest further simplifications. Highlights: Provides a rigorous formalism for 1oo2 system pfd claims. Novel Bayesian approach requires minimal prior information from assessors. System reliability claims are guaranteed to be conservative. Quasi-perfection idea improves on previous perfection models. Avoids pitfalls of naïve and informal approaches.