Robust Resilience: Metaphor and Meaning in Assessing System Performance Ranges

The current emphasis on Resilience Week and the International Symposium on Resilient Cognitive Systems highlights a growing awareness of the importance of designing and operating engineering systems under a variety of environmental conditions and in response to dynamic events. Although there has been considerable confusion and drift in the use of the term, ‘‘resilience’’ as a concept dates back to dynamic systems study of complex ecological systems in the 1970s. This original definition relates clearly to quantitative metrics that link also to statistical process control techniques describing system performance as affected by external, ‘‘assignable’’ causes. This paper discusses important elements in the consideration of resilience as a quantitative metric to improve consistency and clarity of evaluation in engineering systems. Rather than simply a binary attribute of systems, resilience should be considered in terms of system performance measures as affected by environmental conditions or events, energy flow couplings, and statistical process control limits. Our estimations of system resilience are seriously compromised when process control estimates are extrapolated beyond linear ranges of environmental conditions or when including discontinuous performance/event outliers exceeding appropriate forecasting estimates.