The Swiss Cheese model of accident causation is a model used in the risk analysis and risk management of human systems. It likens human systems to multiple slices of Swiss cheese, stacked together, side by side. It was originally propounded by British psychologist James T. Reason in 1990, and has since gained widespread acceptance and use in healthcare, in the aviation safety industry, and in emergency service organizations. It is sometimes called the cumulative act effect.
Reason hypothesizes that most accidents can be traced to one or more of four levels of failure: Organizational influences, unsafe supervision, preconditions for unsafe acts, and the unsafe acts themselves. In the Swiss Cheese model, an organization's defences against failure are modelled as a series of barriers, represented as slices of Swiss cheese. The holes in the cheese slices represent individual weaknesses in individual parts of the system, and are continually varying in size and position in all slices. The system as a whole produces failures when all of the holes in each of the slices momentarily align, permitting (in Reason's words) "a trajectory of accident opportunity", so that a hazard passes through all of the holes in all of the defenses, leading to a failure.
The Swiss Cheese model includes, in the causal sequence of human failures that leads to an accident or an error, both active failures and latent failures. The former concept of active failures encompasses the unsafe acts that can be directly linked to an accident, such as (in the case of aircraft accidents) pilot errors. The latter concept of latent failures is particularly useful in the process of aircraft accident investigation, since it encourages the study of contributory factors in the system that may have lain dormant for a long time (days, weeks, or months) until they finally contributed to the accident. Latent failures span the first three levels of failure in Reason's model. Preconditions for unsafe acts include fatigued air crew or improper communications practices. Unsafe supervision encompasses such things as, for example, two inexperienced pilots being paired together and sent on a flight into known adverse weather at night. Organizational influences encompass such things as reduction in expenditure on pilot training in times of financial austerity.
The same analyses and models apply in the field of healthcare, and many researchers have provided descriptive summaries, anecdotes, and analyses of Reason's work in the field. For example, a latent failure could be the similar packaging of two different prescription drugs that are then stored close to each other in a pharmacy. Such a failure would be a contributory factor in the administration of the wrong drug to a patient. Such research has led to the realization that medical error can be the result of "system flaws, not character flaws", and that individual greed, ignorance, malice, or laziness are not the only causes of error.
Lubnau, Lubnau, and Okray apply Reason's Swiss Cheese model to the engineering of human systems in the field of firefighting, with the aim of reducing human errors by "inserting additional layers of cheese into the system", namely the techniques of Crew Resource Management.