Causal risk models of air transport - NLR-ATSI

Chapter 3. Fundamentals of causation and probability
The subject of this research is causal risk modelling. A causal risk model describes,
probabilistically, the causal relations between certain parameters and risk. To better
understand these models and how they can be used for risk reduction and safety
management it is necessary to describe ‘causation’ and some of its characteristics. To do
that, we must also briefly touch some aspects of probability theory. Their consequences for
the feasibility of building causal risk models will be summarised as a set of requirements
and issues to be resolved.
3.1. What is causation?
Intuitively, we think of a cause as “something that can bring about an effect or result”.
Cause and effect are inseparable. Without some kind of effect the word cause has no
meaning. However, the causes of an effect may not always be evident. This can be the
result of our lack of knowledge, but there is also a second, more subtle characteristic that
can make discussions about cause and effect rather complicated. Consider the example of a
forest fire, which can be caused by, say, lightning or a match. In both cases however, there
would be no forest fire without the oxygen that is in the air. Is it therefore correct to say that
the oxygen is a cause of the fire? [Halpern & Pearl 2001]. Any fire requires three elements:
Combustible material, oxidizer and heat. From the viewpoint of fire physics, all three
elements are equally important: remove any one of these three elements and the fire will not
start, or will die-out (Figure 3).
OXYGEN IGNITION
FUEL
Figure 3: Fire triangle.
In the example of the forest fire, the lightning strike or the match is the final element that
completes the fire triangle and as such is the initiator of the event. In legal terms the dried
leaves and wood of the forest and the oxygen each are a causa sine qua non and the match
is a causa causans. Most people think of ‘cause’ in terms of causa causans. This is not
something new. In ancient Greek the word αιτια not only means cause, but also guilt and
accusation [Muller & Thiel 1986]. When the objective is to assign blame for the forest fire,
the difference between causa sine qua non and causa causans is considered relevant, the
responsible is the causa causans. When the objective is to extinguish the fire, the difference
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