Basic notes on statistics for probabilistic service life predictions

Everybody has some notion what is meant by the term probability. Answers like 'probably yes' or 'probably no' are part of the daily language. The word 'probably' indicates that we are not completely certain about the present status or future development of the item under discussion: two or more options are still open and in the best case we have only some idea that one option is more likely than the other. In every day speech we may even attach numbers to the probability, for instance by saying that the probability of something to happen is about 'fifty fifty'. The purpose of the theory of probability is to enable calculations in this field. In such a theory words like probability and likelihood have received a more formal meaning. However the real meaning and interpretation of probabilities and probabilistic calculations as such is not a part of the theory: two people may have completely different interpretations of the probability concept, but still use the same calculus