Swiss cheese model

The Swiss cheese model, or the Swiss cheese model of accident prevention, is a visual model used by safety professionals to analyze the causes of accidents and adverse events that take place in complex systems such as health care and aviation. The model shows layers of “Swiss cheese” between an adverse incident and management decisions in which many holes open and close at random, with accidents occurring when holes line up on all layers.

Since its introduction in 1990 by James Reason, this landmark in accident prevention research has become the dominant template in the analysis of safety failures as well as studies on the prevention of accidents. The influence of Reason’s work reaches to industries such as transportation, workplace safety, public health, fraud prevention, and nuclear power.

Background

As represented in the Swiss cheese model, Reason’s theory distinguishes the actions taken that are the direct cause of accidents, or “unsafe acts,” from latent organizational conditions that led to that action. These conditions are called latent because they go unnoticed and undetected, sometimes even after they have caused an accident. The idea is that accidents result from the cumulative effect of individuals’ mistakes along with the failings of the entire system. To prevent accidents from happening again, investigators must study not just one individual’s mistake but the problems that exist in the entire system.

Since this theory has been widely adopted, there has been a shift from examining individuals’ errors to the search for latent organizational explanations that led to the final error or action, which Reason calls the “unsafe act.” Wide acceptance of this theory has led accident investigators and safety professionals to focus their attention on potential failures in all levels of complex systems. They now focus on creating safer systems rather than preventing individual mistakes. They carefully examine interrelationships and processes.

Reason himself later challenged the acceptance of the Swiss cheese model and its focus on latent conditions that are far removed from the actual events of an accident. He and other critics have argued that in some cases, unsafe actions are in fact the dominant contributing factor, and changes should be made to increase tolerance for such errors.

Overview

The Swiss cheese model is a visualization of layers in a complex system in which holes open and close. Accidents and adverse events occur when open holes align through all layers. If an arrow is drawn through these aligned holes, it will show how hazards at the top lead to losses at the bottom.

The human factors analysis and classification system (HFACS), developed by Scott Shappell and Doug Wiegmann in the early 2000s, is heavily based on Reason’s Swiss cheese model. The HFACS identifies human error at four levels of failure: unsafe acts of operators; preconditions for unsafe acts; unsafe supervision; and organizational influence. According to this model, in order for an adverse effect—the accident—to occur, one failure must occur at each level. If one of the failures is corrected, the adverse effect is prevented.

The Swiss cheese model calls for a more systems-oriented approach to safety that considers more complex relationships between safety-related events. It does not use a linear, unidirectional approach to understanding cause-effect relationships.

Instead of levels, the Swiss cheese model consists of layers. The innermost layer is the unsafe act. It can include unintentional error and willful disregard for rules and policies. The second innermost layer is the precondition for unsafe acts. This can include the mental condition of the employees, poor communication and coordination practices, poor management of human resources, and poor personal readiness for a task. The next layer is that of supervision. Failures in this layer occur due to inadequate supervision, planned operations that are inappropriate, failure to correct problems when identified, and violations committed by supervisors.

The outermost or top later of the Swiss cheese model is the most removed from the adverse incident. It is the organization itself. Organizational failures can fall into the categories of resource management (including the management of human resources), budgets, and facilities and equipment. It can also include the organizational climate, including the structure of the chain of command and the norms, rules, values, and beliefs that make up the company culture. It might include organizational processes such as operations, procedures, and the presence of risk-management and safety programs.

The Swiss cheese model can be used to analyze the cause of the Zika virus outbreak in Brazil in 2015 and 2016. Zika is spread among human populations by mosquitoes. In this analysis, the top layer is the creation of an environment suitable for mosquitoes. Deforestation and urbanization create fertile environments for mosquitos to breed. On the next layer, political and social factors have led to the return of mosquitoes into populated areas where they had previously been eradicated. On the next layer, large populations of people who have never been exposed to the virus create conditions for the virus to circulate widely for long periods of time. Finally, the virus is introduced to new places through human actions such as air travel. Similarly, in 2020 and throughout the COVID-19 pandemic, healthcare experts, officials, and analysts reflected on the Swiss cheese model, noting that social distancing, the wearing of masks, hand-washing, testing and tracing, ventilation, and government messaging could all work together to prevent further or greater outbreaks.

The model has also been applied to cybersecurity as a means of preventing cyber and malware attacks. In an environment that is constantly changing, companies are susceptible to attacks and must attempt to foresee potential threats. Furthermore, as no single layer is perfect, companies and government agencies must note their vulnerabilities and apply a combination of technical, procedural, and human security controls in order to prevent attacks.

Bibliography

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Perneger, Thomas V. “The Swiss Cheese Model of Safety Incidents: Are There Holes in the Metaphor?” BMC Health Services Research, vol. 5, Nov. 2005, pp. 71. doi.org/10.1186/1472-6963-5-71. Accessed 27 Nov. 2024.

Reason, J. “The Contribution of Latent Human Failures to the Breakdown of Complex Systems.” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, vol. 327, no. 1241, 1990, pp. 475–84. doi:10.1098/rstb.1990.0090. Accessed 27 Nov. 2024.

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Simas, Zach. “How We Use the Swiss Cheese Model to Prevent Malware Infections.” Emsisoft, 16 Feb. 2024, www.emsisoft.com/en/blog/38186/how-we-use-the-swiss-cheese-model-to-prevent-malware-infections/. Accessed 27 Nov. 2024.

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