The total risk is the expected loss: the sum of the products of the consequences multiplied by their probabilities.
Probabilistic risk assessment usually answers three basic questions: Two common methods of answering this last question are event tree analysis and fault tree analysis – for explanations of these, see safety engineering.
Then it was translated in a safety goal for nuclear power plants:[2] The second point is a possible lack of design in order to prevent and mitigate the catastrophic events, which has the lowest probability of the event and biggest magnitude of the impact,[2] and the lowest degree of uncertainty about their magnitude.
Such external events may be natural hazard, including earth quake and tsunami, fire, and terrorist attacks, and are treated as a probabilistic argument.
[2] Changing historical context shall condition the probability of those events, e.g. a nuclear program or economic sanctions.