In this century society faces increasingly large-scale accidents and risks emerging from our own wondrous technologies. Two prominent organizational approaches to safety, Normal Accident Theory and High Reliability Organizations, have focused attention on a variety of industries that deal with hazardous situations, developed concepts to explicate organizational structure and culture, and debated whether accidents are inevitable in complex systems. We outline these approaches and identify some limitations, including narrow definitions, ambiguity about key concepts, confusion of reliability and safety, and overly pessimistic or optimistic conclusions. We believe that the debate between NAT and HRO can become a more productive three-way conversation by including a systems approach to safety emerging from engineering disciplines. The more comprehensive systems approach clarifies the strengths and weaknesses of NAT and HRO and offers a more powerful repertoire of analytic tools and intervention strategies to manage and control post modern risk in complex, high-tech, systems with their potential for catastrophic disruptions and losses.

Resilience is the ability of a system to react to and recover from disturbances with minimal effect on its dynamic stability. While resilience has been the focus of research in several fields, in the case of systemsof-systems (SoSs), addressing resilience is particularly interesting and challenging. As infrastructure SoSs, such as power, transportation, and communication networks, grow in complexity and interconnectivity, measuring and improving the resilience of these critical SoSs is vital in terms of safety and providing uninterrupted services. While the resilience of SoSs depends on the reliability of their constituent systems, traditional reliability and risk assessment approaches cannot adequately quantify their resilience. In this paper, we provide an evaluation of the various methods available and challenges associated with designing resilient SoSs by (1) indicating important differences between resilience and various related system attributes, (2) providing a critical assessment of the current reliability and risk techniques in addressing SoS resilience, and (3) discussing the application of recent multidisciplinary research that can guide the design of resilient SoS. Finally, we highlight key challenges in this design process and propose a series of research themes that can shape future research in this field. C⃝ 2015 Wiley Periodicals, Inc. Syst Eng 18: 491-510, 2015

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