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course, means that 1% are violent, and as there are roughly
1,000,000 Londoners between the ages of 15 and 25, a little
reflection suggests there are 10,000 seriously violent young
people running around — not the image the communicators
wanted to conjure up.
It is generally argued that using relative measures is a
manipulative way of communicating risk, because it has ben
shown to increase the apparent importance of a particular
action. Being told that regularly eating a bacon sandwich
increases your lifetime risk of pancreatic cancer by 20% may
be somewhat unconvincing if the baseline is extremely low
and the bacon sandwich is rather pleasant.
However, this situation can be reversed in acute lowprobability,
high-impact events that occur regularly: we
all take daily precautions when travelling, for example,
which makes small risks even smaller. This point was also
dramatically illustrated when Italian earthquake advisors
were convicted of involuntary manslaughter for issuing
unduly reassuring messages to the residents of L’Aquila in
2009, a few days before more than 300 people were killed
in a major earthquake. The advice correctly said that the
overall risk was low, but should also have emphasized that
the relative risk was high: this would have enabled residents
to adopt their own chosen level of precaution (see case
study in Chapter 1 for further discussion of L’Aquila).
Clearly both absolute and relative risks are required. The
important lesson from numerical risk communication is that
one size does not fit all, and a variety of methods may be
appropriate, with a hierarchy of numerical sophistication.
3. Using words
It is important to realize that words such as ‘likely’ or
‘possible’ carry meaning beyond mere magnitude, and
depends crucially on context. For example, the UK’s
Terrorism Threat Level scale defines SEVERE as meaning
“that a terrorist attack is highly likely”, and yet when this
CASE STUDY
NUCLEAR: THE SUBMARINER’S PERSPECTIVE
Rear Admiral Nigel Guild (Chairman, Engineering Council)
A
nuclear submarine is one of
the most complex engineering
achievements known to man,
and it contains a unique combination
of potential hazards within a relatively
small space. These hazards include structural and
environmental issues common to all large ships, with
the added problem that the vessel needs to remain
stable under the water. To these challenges are added
nuclear propulsion, explosives and, in the case of the
deterrent submarine, nuclear weapons.
The management of submarine safety is critical to
the protection of submariners, the public and the
environment. The Royal Navy’s approach to managing
the risks presented by these potential hazards is
to assess and mitigate them through the use of a
probabilistic safety case. This safety case aggregates
the assessment of risks, so that there is ultimately
an overview of risk for the whole submarine. This
process relies on normal definitions of acceptable
risk used by the Health and Safety Executive (HSE),
along with widely used diagrams depicting the
expectation of death per year against the number of
people employed. HSE Basic Safety levels are used for
the maximum risk that is normally allowable. Then
the ‘As Low As Reasonably Practicable’ (ALARP)
principle is deployed to continually reduce risk from
each potential hazard, until the cost of further effort
would be grossly disproportionate to the extra safety
achieved.
In practice, far greater resources are devoted to
managing nuclear safety than for other potential
submarine hazards with the same risk assessment. This
is required by a public expectation of far greater risk
reduction for a potential nuclear hazard, because it is
not generally understood and it is held in significant
dread. To take a non-nuclear example, the risk of a
seamanship accident, such as falling into the sea while
working on the casing when the submarine is on the
surface, is assessed in a similar way to any workplace
potential hazard. In contrast to this, a potential nuclear
event requires risk mitigation to achieve two orders
of magnitude smaller risk assessment than would
be sought for conventional risks. Another way of
expressing this is by applying the ALARP principle: the
effort required before it would be considered grossly
disproportionate to the extra nuclear safety achieved
is about 100 times more than for other risks.
Using this logical approach, a consistent set of
safety assessments for the whole submarine can
be assembled and used to minimize risks using the
common language of health and safety assessment.
Within the process, however, chosen risks such as
nuclear can be managed to different levels of ALARP
in accordance with society’s expectation.
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