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The
risk associated with a given event is a function of both
the likelihood and impact of such an event. The usual
definition of risk is: Risk = likelihood x impact. Risks
are often plotted on a logarithmic scale, thus:
Log (Risk) = Log (Likelihood) + Log (Impact magnitude)
In a log-log diagram, the constant risk loci given by
the above equation become straight lines, as shown in
Figure 2. Since it is not always possible or convenient
to develop quantitative models for both probability and
consequences, you can estimate their respective magnitudes,
for example in scales from 1 to 4, and assign levels of
risk to the different combinations, as shown in Figure
3.
The high likelihood and high magnitude position corresponds
to the highest risk (IV). Risk levels are defined for
each situation. The disaster at the Phillips
66 plant poignantly demonstrates the relationships
of probability and consequences.
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