Swiss Cheese Model
Reason proposed what is referred to as the “Swiss Cheese Model” of system failure. Every step in a process has the potential for failure, to varying degrees. The ideal system is analogous to a stack of slices of Swiss cheese. Consider the holes to be opportunities for a process to fail, and each of the slices as “defensive layers” in the process. An error may allow a problem to pass through a hole in one layer, but in the next layer the holes are in different places, and the problem should be caught. Each layer is a defense against potential error impacting the outcome.
For a catastrophic error to occur, the holes need to align for each step in the process allowing all defenses to be defeated and resulting in an error. If the layers are set up with all the holes lined up, this is an inherently flawed system that will allow a problem at the beginning to progress all the way through to adversely affect the outcome. Each slice of cheese is an opportunity to stop an error. The more defenses you put up, the better. Also the fewer the holes and the smaller the holes, the more likely you are to catch/stop errors that may occur.
The original source for the Swiss Cheese illustration is: “Swiss Cheese” Model – James Reason, 1991. The book reference is: Reason, J. (1990) Human Error. Cambridge: University Press, Cambridge.