[Ed note: This is the first post in a series documenting key concepts and research from the field of Resilience Engineering. Read more about the series here.]

Resilience Engineering is the science of studying the presence of and means to cultivate resilience. Though a widely used term, the definition of "resilience" is not equally shared. In his paper Four concepts for resilience and the implications for the future of resilience engineering, Dr. David Woods gives us four variations as commonly used, so we can better distinguish what we mean when using these terms.

1. Resilience as Rebound

One form of resilience is the ability for a system to rebound to its preferred state or a different state in which it is able to perform at a desired level. When stressors are applied to a system, the system is resilient if it is able to recover. In this sense, it's looking at what capabilities and resources the system possesses before the time of the impacting events that allow it to then return.

2. Resilience as Robustness

Resilience can also be characterized as robust when it has the ability to absorb or withstand stressors. When perturbations impact a system, the system in turn resists the change in order to continue functioning in expected ways. A system improves when it grows the range of disturbances that it can experience without significant failure.

3. Resilience as Graceful Extensibility

When a system is able to continue beyond expected parameters by adapting to novel stressors, it's said to exhibit graceful extensibility. The success of the system is understood in terms of possessing finite resources and then its ability to stretch so that those resources are used in novel ways to achieve its goals.

4. Resilience as Sustained Adaptability

Sustained Adaptability centers itself on understanding systems as layered networks. These systems have succeeded and will continue to do so if the connective ability of multiple strata are able to work cohesively—shifting resources, understanding and applying the inherent trade-offs, and how the human element of the system decides where, when, and how to make those trade-offs.

While the first two elements have been studied for a long time already, Resilience Engineering looks at the system through the lens of graceful extensibility and sustained adaptability. It's the adaptive elements of the system that can respond to the unexpected. In doing so, we create environments to allow for continued success.

References

• Four concepts for resilience and the implications for the future of resilience engineering
• Three Takes on Four Concepts for Resilience Engineering

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