Accident causation, Swiss cheese

The Swiss cheese model of Reason, shown here, is a graphical
presentation of accident causation. It enables us to
understand the difficulties companies encounter in preventing
hazards from becoming risks, thus creating losses,
or damage to humans and/or material. The different slices
of the cheese represent the layers of defences, like barriers
or safeguards, a company has installed as part of its risk
prevention program. The model refers to long latency periods
of the so-called latent conditions. This is represented
by the holes in the first slices. Examples of latent conditions
are poor design, gaps in supervision, undetected manufacturing
defects, defects or maintenance failures, unworkable
procedures, clumsy automation, shortfalls in training, or
less than adequate tools and equipment. Like pathogens in
the human body, latent conditions may be present for many
years before they combine with local circumstances and
activate failures to penetrate the many layers of defences.
Unsafe acts are mostly situated in the last slices, while latent
conditions are the holes throughout the cheese.