The Swiss Cheese Model

Organizations often have vulnerabilities, and professionals work to create defenses against potential failures. If a mistake goes unnoticed at multiple stages, it can lead to a catastrophe. This phenomenon is known as the "Swiss Cheese Model" or the "Cumulative Act Effect," as explained by James Reason of the University of Manchester.

In the Swiss Cheese Model (SCM), the slices of cheese are identified as defense mechanisms which prevent hazards from occurring. Each defense mechanism has unintended weaknesses which are depicted by the holes in cheese. These holes represent the individual weaknesses in individual parts of the system, and continually vary in sizes and positions. If by any chance a mistake slips through all these holes, across all the layers of cheese when all the holes are in alignment, a catastrophe might result.

These holes may appear due to "active errors" or "latent conditions”. Active errors have an immediate consequence and are usually made by front-line people. These are the unsafe acts executed by people who are in direct contact with the system.

Active errors have a direct and usually short-lived impact on the integrity of the defenses. 

Latent conditions are usually hidden within an organization until they are revealed by an event which is likely to have serious consequences. They arise from decisions made by designers, builders, procedure writers and the Top-Level Management.

Latent conditions can lead to two kinds of adverse effects.

1. They can translate into error inducing conditions within the local workplace. 

Eg: time pressure, under-staffing, inadequate equipment, fatigue, and inexperience 

2. They can create long lasting weaknesses in the defenses. 

Eg: untrustworthy alarms and indicators, unworkable procedures, design and construction deficiencies

 These failures can be highlighted and corrected via effective risk management before problems manifest in the system. Incidents within an organization may arise due to a combination of both latent conditions and active failures. 

What can we learn from the SCM?

• Add Slices 

We can include more defense layers to prevent a disaster, but it is equally important to study how these layers interact. Here, we have to carefully analyze the system to determine if there is an “Option B”. 

Failures can arise from any section of an organization and they might be integrated. A failed system implementation may result from multiple reasons like poor project management, inaccurate requirements, inadequate training, etc. Thus, organizational problems or failures need to be thoroughly analyzed and addressed from different angles. Hence, they can be resolved completely and provide learning opportunities for project teams. 

• Fill the holes 

Most often, the best solution is to minimize the holes in each layer by making them more robust or to replace a layer with one that better addresses all of the risks. The classic SCM has changed over time to introduce a modified model called the Hot Cheese Model. 

Hot Cheese Model: A Processed Swiss Cheese Model 

The SCM has gained success within industries for many years. However, its graphical representation may be misleading. System defenses are visualized as a collection of unchanging, independent features. 

The model seems to suggest that as long as enough layers of defenses are in place, at least ones with non-aligning holes, the probability of an accident can be minimized. The truth is, system defenses are more active. 

There have been instances when a defense layer was introduced in the hope of preventing errors and improving system performance, but only ended up emerging new errors and causing more harm.

Three key contributors to incidents in HCM 

• Force : The combination of active errors allowed forward and risks introduced by the system defense layers upstream. 
• Loophole : A latent condition hidden in a layer that may allow force to fall through. 
• Drip : New risks introduced by the current system defense layer due to design.

The new model creatively covers more issues than the original Swiss Cheese Model, while retaining its strengths. Most importantly, hot cheese — since it drips! — vitalizes more perceptive discussion than the original static Swiss Cheese Model.