Swiss cheese model

File:Emmentaler aoc block.jpg with eyes. When cut into slices, each slice will have holes of varying sizes and positions.]]

In the Swiss cheese model, an organization's defenses against failure are modeled as a series of imperfect barriers, represented as slices of cheese, specifically Swiss cheese with holes known as "eyes", such as Emmental cheese. The holes in the slices represent weaknesses in individual parts of the system and are continually varying in size and position across the slices. The system produces failures when holes in each slice momentarily align, permitting (in Reason's words) "a trajectory of accident opportunity",{{Cite book |last=Reason |first=James |title=Human Error |publisher=Cambridge University Press |year=1990 |isbn=978-0-521-30669-0 |location=New York, N.Y. |language=en}} so that a hazard passes through holes in all of the slices, leading to a failure.{{cite book|title=Controlling Pilot Error|author1=Daryl Raymond Smith |author2=David Frazier |author3=L W Reithmaier |author4=James C Miller |name-list-style=amp |pages=10|year=2001|publisher=McGraw-Hill Professional|isbn=0071373187}}{{cite book|pages=4–6|title=Clinical Risk Management in Midwifery: the right to a perfect baby?|author1=Jo. H. Wilson |author2=Andrew Symon |author3=Josephine Williams |author4=John Tingle |name-list-style=amp |year=2002|publisher=Elsevier Health Sciences|isbn=0750628510}}{{cite book|title=Clinical Governance in Mental Health and Learning Disability Services: A Practical Guide|editor1=Adrian J. B. James |editor2=Tim Kendall |editor3=Adrian Worrall |pages=176|year=2005|publisher=Gaskell|isbn=1904671128|chapter=Risk management |author1=Tim Amos |author2=Peter Snowden |name-list-style=amp }}{{cite book|title=Human Factors and Behavioural Safety|author=Stranks, J. |pages=130–31|publisher=Butterworth-Heinemann|date=2007|isbn=9780750681551}}

Frosch{{cite book|title=Seeds of Disaster, Roots of Response: How Private Action Can Reduce Public Vulnerability|editor1=Philip E Auerswald |editor2=Lewis M Branscomb |editor3=Todd M La Porte |editor4=Erwann Michel-Kerjan |author=Robert A. Frosch|pages=88|chapter=Notes toward a theory of the management of vulnerability|year=2006|publisher=Cambridge University Press|isbn=0521857961}} described Reason's model in mathematical terms as a model in percolation theory, which he analyses as a Bethe lattice.

The model includes active and latent failures. Active failures encompass the unsafe acts that can be directly linked to an accident, such as (in the case of aircraft accidents) a navigation error. Latent failures include contributory factors that may lie dormant for days, weeks, or months until they contribute to the accident. Latent failures span the first three domains of failure in Reason's model.{{cite book |last1=Wiegmann |first1=Douglas A. |title=A Human Error Approach to Aviation Accident Analysis: The Human Factors Analysis and Classification System |last2=Shappell |first2=Scott A. |publisher=Ashgate Publishing |year=2003 |isbn=0754618730 |pages=48–49 |language=en}}

In the early days of the Swiss cheese model, late 1980 to about 1992, attempts were made to combine two theories: James Reason's multi-layer defence model and Willem Albert Wagenaar's tripod theory of accident causation. This resulted in a period in which the Swiss cheese diagram was represented with the slices of cheese labelled 'active failures', 'preconditions' and 'latent failures'.{{Citation needed|date=June 2025}}

The attempts to combine these theories still cause confusion today. A more correct version of the combined theories is shown with the active failures (now called immediate causes), preconditions and latent failures (now called underlying causes) shown as the reason each barrier (slice of cheese) has a hole in it, and the slices of cheese as the barriers.{{Citation needed|date=June 2025}}

File:Covid-19-Cheese-Model-animation-02-short.gif{{cite web | author-link = Siouxsie Wiles |last1=Wiles |first1=Siouxsie |title=Siouxsie Wiles & Toby Morris: Covid-19 and the Swiss cheese system |url=https://thespinoff.co.nz/science/22-10-2020/siouxsie-wiles-toby-morris-covid-19-and-the-swiss-cheese-system/ |website=The Spinoff |access-date=28 October 2020 |date=22 October 2020}}]]

The framework has been applied to a range of areas including aviation safety, various engineering domains, emergency service organizations, and as the principle behind layered security, as used in computer security and defense in depth.{{cite book|title=Enhancing Occupational Safety and Health|date=2004|last1=Taylor |first1=G. A. |last2=Easter |first2=K. M. |last3=Hegney |first3=R. P. |publisher=Elsevier |isbn=0750661976 |pages= 140–41, 147–53, 241–45}}

The model was used in some areas of healthcare. For example, a latent failure could be the similar packaging of two drugs that are then stored close to each other in a pharmacy. This failure would be a contributory factor in the administration of the wrong drug to a patient. Such research led to the realization that medical error can be the result of "system flaws, not character flaws", and that greed, ignorance, malice or laziness are not the only causes of error.{{cite book |author1=Patricia Hinton-Walker |title=Annual Review of Nursing Research Volume 24: Focus on Patient Safety |author2=Gaya Carlton |author3=Lela Holden |author4=Patricia W. Stone |date=2006-06-30 |publisher=Springer Publishing |isbn=0826141366 |editor1=Joyce J. Fitzpatrick |pages=8–9 |chapter=The intersection of patient safety and nursing research |editor2=Patricia Hinton-Walker |name-list-style=amp}}

The Swiss cheese model is nowadays widely used within process safety. Each slice of cheese is usually associated to a safety-critical system, often with the support of bow-tie diagrams. This use has become particularly common when applied to oil and gas drilling and production, both for illustrative purposes and to support other processes, such as asset integrity management and incident investigation.{{Cite book |last=CCPS in association with Energy Institute |title=Bow Ties in Risk Management: A Concept Book for Process Safety |publisher=AIChE and John Wiley & Sons |year=2018 |isbn=9781119490395 |location=New York, N.Y. and Hoboken, N.J. |language=en}}

Lubnau, Lubnau II, and Okray apply the model to the engineering of firefighting systems, aiming to reduce human errors by "inserting additional layers of cheese into the system", namely the techniques of Crew Resource Management.{{cite book|title=Crew Resource Management for the Fire Service|author1=Thomas Lubnau II |author2=Randy Okray |author3=Thomas Lubnau |name-list-style=amp |year=2004|publisher=PennWell Books|isbn=1593700067|pages=20–21}}

Olson and Raz apply the model to improve deception in the methodology of experimental studies, with multiple thin layers of cheese representing subtle components of deception which hide the study hypothesis.{{Cite journal |last1=Olson |first1=Jay A. |last2=Raz |first2=Amir |date=2021 |title=Applying insights from magic to improve deception in research: The Swiss cheese model |url=https://linkinghub.elsevier.com/retrieve/pii/S0022103120303930 |journal=Journal of Experimental Social Psychology |language=en |volume=92 |pages=104053 |doi=10.1016/j.jesp.2020.104053|s2cid=228919455 |url-access=subscription }}

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• Chain of events (accident analysis)
• Healthcare error proliferation model
• Iteration
• Latent human error
• Mitigation
• Proximate and ultimate causation
• Proximate cause
• Redundancy (engineering)
• Root cause analysis
• System accident
• Systems engineering
• Systems modelling

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{{Reflist|2}}

Category:Aviation safety

Category:Error

Category:Failure

Category:Metaphors referring to food and drink

Category:Process safety

Category:Safety engineering

Category:Scientific models