Probability is a mathematical assessment of the frequency of specific events. Unfortunately it is not static. Historical data provides a retrospective view of probability. However, if there are significant changes in the environment, you can expect probabilities to change. If there are no nuclear power plants in a locality, the potential of a serious nuclear power plant accident within the community is very, very small. Build a nuclear power plant and that changes. More pavement to accommodate more housing developments and shopping areas means an increased probability of urban runoff flooding. Deferred or no maintenance by an airline in financial difficulties means an increased probability of major accidents among the airline’s fleet.