Marvin Rausand - Risk Assessment

The quotation from Jaynes (2003) touches on another controversy: that is, whether the probability of an event is a property of the event , the experiment producing , or a subjective probability that exists only in the individual's mind.

The mathematical rules for manipulating probabilities are well understood and are not controversial. A nice feature of probability theory is that we can use the same symbols and formulas whether we choose the frequentist or the Bayesian approach, and whether or not we consider probability as a property of the situation. The difference is that the interpretation of the results is different.

Some researchers claim that the frequentist approach is objective and therefore the only feasible approach in many important areas, for example, when testing the effects of new drugs. According to their view, such a test cannot be based on subjective beliefs. This view is probably flawed because the frequentist approach also applies models that are based on a range of assumptions, most of which are subjective.

When an event occurs more or less frequently, we often talk about the frequency of rather than the probability of . Rather than asking “What is the probability of event ,” we may ask, for example, “How frequently does event occur?”

Fatal traffic accidents occur many times per year, and we may record the number of such accidents during a period of length . A fatal traffic accident is understood as an accident where one or more persons are killed. The frequency of fatal traffic accidents in the time interval is given by

(2.6)

The “time” may be given as calendar time, accumulated operational time (e.g. the accumulated number of hours that cars are on the road), accumulated number of kilometers driven, and so on.

In some cases, we may assume that the situation is kept unchanged and that the frequency approaches a constant limit when . We call this limit the rate of the event and denote it by :

(2.7)

In the frequentist interpretation of probability, parameters like have a true, albeit unknown value. The parameters are estimated based on observed values, and confidence intervals are used to quantify the variability in the parameter estimators.

Models and formulas for the analysis may be found in Appendix A.

The third question in our definition of risk introduces the term consequences . A consequence involves specific damage to one or more assets and is also called adverse effect, negative effect, impact, impairment , or loss .

ISO Guide 73 defines consequence as “outcome of an event affecting objectives,” in line with their definition of risk. The term harm is used in several important standards, including ISO 12100. Their definition of harm is “physical injury or damage to health,” limiting the definition only to cover people. This is in line with the objectives of the standard. In this book, no distinction is made between harm and consequence, and we define it more generally as follows:

Injury or damage to the health of people, or damage to the environment or other assets.

To answer the third question in the triplet definition of risk “what are the consequences?” we first have to identify who – or what – might be harmed. In this book, these “objects” are called assets .

Something we value and want to preserve.

Assets are also called targets, vulnerable targets, victims, recipients, receptors , and risk‐absorbing items . Examples of assets are listed in Table 2.6. Observe that the sequence of the assets in Table 2.6does not imply any priority or ranking.

Table 2.6Some types of assets.

In most risk assessments, humans are considered to be the most important asset. The possible victims of an accident are sometimes classified according to their proximity to and influence on the hazard (Perrow 1984):

1 (1) First‐party victims. These are people directly involved in the operation of the system. Employees in a company where accidents may occur are typical examples of first‐party victims.

2 (2) Second‐party victims. These are people who are associated with the system as suppliers or users, but exert no influence over it. Even though such exposure may not be entirely voluntary, these people are not innocent bystanders, because they are aware of (or could be informed about) their exposure. Passengers on airplanes, ships, and railways, for example, are considered to be second‐party victims.