NAT and HRO; Summary

The NAT and HRO theories both simplify the cause of accidents. HRO underestimates the problems of uncertainty. NAT recognizes the difficulty of dealing with uncertainty but underestimates and oversimplifies the potential ways to cope with uncertainty. Both theories believe that redundancy is the only way to handle risk.

Limitations of Both NAT and HRO

Perrow contributed with his definition of NAT, by identifying interactive complexity and tight coupling as critical factors which shouldn’t be discounted. His top-down system view of accidents versus the bottom-up, component reliability view of the HRO theorists is critical in understanding and preventing future accidents. While the HRO theorists do offer more suggestions, most of them are inapplicable to complex systems or oversimplify the problems involved.

A top-down, systems approach to safety

First, it is important to recognize the difference between reliability and safety. HRO researchers talk about a “culture of reliability” where it is assumed that if each person and component in the system operates reliably, there will be no accidents.

Highly reliable systems are not necessarily safe and highly safe systems are not necessarily reliable. Reliability and safety are different qualities and should not be confused. In fact, these two qualities often conflict. Increasing reliability may decrease safety and increasing safety may decrease reliability.

Reliability in engineering is defined as the probability that a component satisfies its specified behavioral requirements over time and under given conditions. If a human operator does not follow the specified procedures, then they are not operating reliably. In some cases that can lead to an accident. In other cases, it may prevent an accident when the specified procedures turn out to be unsafe under the circumstances.

If the goal is to increase safety, then we should be talking about enhancing the safety culture, not the reliability culture. The safety culture is that part of organizational culture that reflects the general attitude and approaches to safety and risk management. Aircraft carriers do have a very strong safety culture and many of the aspects of this culture observed by the HRO researchers can and should be copied by other organizations but labeling these characteristics as “reliability” is misleading and can lead to misunderstanding what is needed to increase safety in complex, tightly coupled systems.

Safety is an emergent or system property, not a component property. Determining whether a plant is acceptably safe is not possible by examining a single valve in the plant (although conclusions can be reached about the valve’s reliability). Safety can be determined only by the relationship between the valve behavior and the other plant components and often the external environment of the plant—that is, in the context of the whole. A component and its specified behavior may be perfectly safe in one system but not when used in another.


Shrivastava, S., Sonpar, K. &Pazzaglia F. (2009) ”Normal accident theory versus High reliability theory: a resolution and call for an open systems view of accidents”, find it here

Marais, K., Dulac, N. & Leveson, N.: ”Beyond normal accidents and hugh reliability organizations: The need for an alternative approach to safety in Complex systems”, MIT find it here


As promised this post is about High reliability organizations. If you haven’t read the latest post about natural accident theory (NAT), we recommend you to, by following this link

Around the same time Perrow articulated his NAT (natural accident theory), another stream of research emerged. Sholars from the Berkely campus of the University of California came together to study how organizations that operate complex, manage to remain accident-free for a longer time. This group and other scholar’s latter became what we know as HRO.

High reliability organizations operate in complex, high-hazard domains, for extended periods without serios accidents or catastrophic failure.  

Just like in the NAT, HRP are related to the two dimensions; interactive complexity and loose/tight coupling – which Perrow claimed together determine a system’s susceptibility to accidents.
Interactive complexity refers to the presence of unfamiliar or unplanned and unexpected sequences of events in a system that are either not visible or not immediately comprehensible. A tightly coupled system is one that is highly interdependent: Each part of the system is tightly linked to many other parts and therefore a change in one part can rapidly affect the status of other parts. Tightly coupled systems respond quickly to perturbations, but this response may be disastrous. Loosely coupled or decoupled systems have fewer or less tight links between parts and therefore can absorb failures or unplanned behavior without destabilization.  An HRO is hypercomplecity – extreme variety of components, system, and levels, combined by a really tight coupling, meaning a reciprocal interdependence across many units and levels.

Why reliability?

Although HRT scholars have abandoned attempts to explicitly define reliability, they appear to agree that reliability is the ability to maintain and execute error-free operations. 

HRO scholars report that HRO emphasis the following conditions as being necessary, but not sufficient, for ensuring reliability: a strategic prioritization of safety, careful attention to design and procedures, a limited degree of trial-and-error learning, redundancy, decentralized decision making, continuous training often through simulation, and strong cultures that encourage vigilance and responsiveness to potential accidents.

Why is this important?

It is important to recognize that standardization is necessary but not sufficient for achieving resilient and reliable health care systems. High reliability is an ongoing process or an organizational frame of mind, not a specific structure. Examples of such organizations could be health care organizations aiming to become highly reliable in their report of practice. Other examples include Air traffic control system, nuclear power plant and NASA. In each case, even a minor error could have catastrophic consequences.

Your organization

Even though your organization might not hold lives in its hands, all organizations still face risks to profits, customer satisfaction, and reputation. The behaviors of HROs can be very instructive for those trying to figure out how to error-proof processes, avoid surprises, and deliver the desired outcome every single time.


Shrivastava, S., Sonpar, K. &Pazzaglia F. (2009) ”Normal accident theory versus High reliability theory: a resolution and call for an open systems view of accidents”, find it here

Marais, K., Dulac, N. & Leveson, N.:” Beyond normal accidents and high reliability organizations: The need for an alternative approach to safety in Complex systems”, MIT find it here