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Fig. 1. Risk assessment classification
This has resulted in classifying risk assessment methods as quantitative,
semi-quantitative and qualitative (figure 1).
In the past, mainly quantitative and qualitative methods were used.
Quantitative methods are time consuming and therefore rather expen-
sive. They aim at a detailed description of scenarios describing how a hazard
can become an accident and looking at all the barriers in between. After the
process is described qualitatively using e.g. event trees, petri nets or markov
chains, quantification has to be done. Unfortunately, in general no data, not
enough or no statistically valid data is known. Therefore, analyses relay heav-
ily on expert's opinion.
Qualitative methods on the other hand are fairly easy to apply. They do
not pretend to result in very exact results but make it obvious that the results
are rather assessed than calculated. However, it is not proven that the results
are significantly different from the results one would obtain with quantitative
methods. A major drawback is that qualitative methods were developed over
the years using expert's opinion. Therefore, it cannot be proven that the
results are actually correct if compared to a tolerable risk.
For the future, hybrid methods, which allow combining the advantages of
qualitative and quantitative risk assessment methods, should be used. These
methods are called semi-quantitative. They are based on a mathematical
model, which allows for a detailed verification of the results and the conditions
taken into account. However, these methods are presented in a qualitative way
which allows for an easy and fast assessment.
The presented guidelines for the construction process can be applied to
any semi-quantitative risk assessment method. The following discussion will
lead to the construction of a semi-quantitative risk graph. The risk graph is a
