5. A reminder of probability

The definitions and methods used in the discipline of operational safety are based on probability theory, which is the subject of the following reminders, based on a concrete notion in operational safety: the random time T at which a failure occurs. However, as real systems have no physical existence for negative times, it is essential to take this particularity into account when redefining the probability laws governing the time of failure. It is therefore absolutely essential never to use negative instants to describe the behavior of a piece of equipment. In the field of operational safety, this essential characteristic must be taken into account, and the reader can find out more about these probability techniques in a number of books: