The swiss cheese model of safety

A recent fence jumper at the White House seems to have made it--incredibly--inside the building a fair ways before being taken down. Reading about the multiple failures across various security levels--alarm boxes being silenced, undercover agents outside missing the climber, attack dogs not being released etc.--I am reminded of the 'swiss cheese model of safety'.

In this model, each security layer can be thought of as a slice of cheese. Because all security measures have flaws, the slices contain holes that allow failure opportunities to slip through. Typically these 'holes' don't line up, so a failure at one level is caught by another level. But since modern security systems are complex and dynamic, occasionally a failure can proceed all the way through the system and become realized.

The key insight of this model is that each security layer contributes only in a probabilistic sense to the overall security system. This presents a challenge to managers because employees operating largely within one 'slice' can easily get complacent and start taking shortcuts (like turning off the alarm boxes because they were loud and annoying), increasing the opportunities for overall failure.