Risk Analysis, 2nd Edition

Appendix A

Probability calculus and statistics

A.1 The meaning of a probability

A probability can be interpreted in different ways. In this book, we understand a probability to be an expression of how likely (credible) it is that an event will occur. Let us look at an example. Let represent the event that a patient develops an illness, , over the next year, when the patient shows symptoms, . We do not know if will occur or not—there is uncertainty associated with the outcome. However, we can have an opinion on how likely it is that the patient will develop the illness. Statistics show that about 5 out of 100 patients develop this illness over the course of 1 year if they show the symptoms . Is it then reasonable to say that the probability that will occur is equal to 5%? Yes, if this is all the information that we have available, then it is reasonable to say that the probability that ...