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Probability is expressed in terms of the planning or operational time horizon for your organization. Every assessment of hazards and risks is essentially time based, whether or not we say it is. The time factor can significantly change your perception of the threat that a specific type of event can pose. For example, if the local volcano erupts on an approximate cycle of once every 200 years, based on geologic evidence of the last 2000 years, its last eruptive cycle finished 10 years ago, and you are planning for the next 5 years, it is reasonable to lower the probability of a volcanic event. However, it is not reasonable to rate this probability as 0.00, as the eruptive cycles of volcanoes are not absolutely predictable, and your volcano may be the one that proves that. Remember also that environmental changes will change probabilities, and that in many cases new understandings of threats or new threats may well emerge during the period for which you are forecasting risk. In other words, probability is not fixed it is dynamic, and must be regularly reconsidered.