Likelihood is the
extent to which risk effects are likely to occur. Likelihood is
not the same thing as probability.
The likelihood of an event often depends not only on blind statistical
chance, but also on human intervention. A common way of expressing
this would be: “I know this is highly likely to happen if
I do nothing, but then I don’t intend to do nothing.”
Intervention Difficulty
For this reason, we divide likelihood into two
components: probability
(the probability that the risk events will occur in the absence
of human intervention) and intervention
difficulty (the level of difficulty that we would experience
in preventing the risk event from occurring). Note that intervention
difficulty does not define response actions, but merely indicates
the availability of such actions.
A typical intervention difficulty scale is shown below.
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Probability
It is tempting to give numerical values to probability. (“What’s
the probability that this supplier will deliver late?” “Oh,
about 30 percent, I’d say.”) But where do such numbers
come from? An estimate of 30 percent would be reasonable if the
supplier had made 10 previous deliveries, three of which were
late. However, if this is only the first or second time that you
have used this supplier, you are in effect trying to calculate
statistics based on a single sample.
For this reason, a nonnumeric probability scale is generally
recommended, such as the one shown (Of course, if you do have
reliable numeric data, use it.)
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Combining Probability and Intervention
Difficulty
To see how probability and intervention difficulty
work together, imagine yourself standing on a railway track. If
you stand there long enough, a train will hit you. The probability
of that occurrence depends on how often trains use the track.
However, the likelihood of being hit also depends on how easy
it is to get off the track. If the track is on flat, open land,
so that you can easily step off when a train approaches (low
intervention difficulty), then the likelihood of being
hit is effectively independent of the frequency of trains. But
if you are in the middle of a long tunnel, then you may not be
able to get off the track in time (high intervention difficulty),
so the likelihood of being hit depends in this case on how often
trains use the track.
This leads to a simple relationship between likelihood, probability,
and intervention difficulty: likelihood is the lower of the two
ratings for probability and intervention difficulty.
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Unique Situations
One advantage of expressing likelihood in this
way is that it can deal with unique situations. If a particular
situation is indeed unique, it is impossible to make any estimate
of its probability of occurrence because, by definition, it has
never happened before.
In such cases, we first decide if the situation could reasonably
occur. If the answer is yes, we assume that it will happen (Level
5 probability) in which case the lower-of rule means that likelihood
has the same rating as intervention difficulty.
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